a geochemical clock in earliest paleogene pelagic carbonates based on the impact induced os isotope excursion at the cretaceous paleogene boundary 1 an impact induced osmium os isotope excursion provides a unique means of assessing the completeness of marine cretaceous paleogene k pg boundary sections and surmounting challenges associated with constraining the time scale of the earth system recovery from this extreme perturbation a model of the recovery of seawater187os 188os following the impact event allows independent estimates of the time elapsed since the impact which can be directly compared to time estimates derived from biostratigraphy magnetostratigraphy and cyclostratigraphy this approach is tested using data from three deep ocean sites cored by the ocean drilling program odp data from odp 1262b south atlantic and odp 690c southern ocean display the expected 187os 188os minimum very close to the biostratigraphically defined k pg boundary and yield os based accumulation rate estimates similar to those obtained from magnetostratigraphy and orbital tuning in contrast the187os 188os minimum in odp 1209c western pacific occurs 9 cm below the k pg boundary low os concentrations throughout the boundary interval and an implausibly rapid recovery to higher pre impact187os 188os ratios provide strong evidence for a previously unrecognized gap in the k pg interval of site 1209 results presented here provide strong empirical evidence that os isotope data are uniquely valuable in assessing the completeness and accumulation rates of earliest paleogene sediments from the deep sea they are of broad interest because they have implications for astronomical tuning of the geologic time scale and illustrate that whole ocean geochemical perturbations can provide an alternative to biostratigraphy for correlation and timekeeping during abrupt biotic events

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

practical estimation of compression behavior of clayey silty sands using equivalent void ratio concept abstractclayey silty sands are widespread as naturally sedimentary soils such as marine deposits in estuaries and offshore locations they belong to a unique class of gap graded soils featuring a d

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

marine os isotopic fluctuations in the early eocene greenhouse interval as recorded by metalliferous umbers from a tertiary ophiolite in japan abstract the mineoka umbers occur within the tertiary accretionary complex of central japan immediately overlying 53 7 ma tholeiitic basalts the associated tholeiitic basalts exhibit typical normal mid ocean ridge basalt n morb type geochemical signatures and are interpreted to be a remnant of tertiary oceanic crust centimeter resolution bulk rock chemical analyses were performed to cover the entire section of the 35 cm thick umber deposit there are striking similarities in major element trace element re and os contents between the umbers and modern metalliferous sediments along the east pacific rise these results demonstrate that the mineoka umbers are ancient counterparts of mor metalliferous sediments that are plume fall out ferromanganese precipitates scavenging various seawater derived elements including p v y rees and os and confirm that the umbers subaerially exposed in the accretionary complex generally preserve their primary geochemical signatures at the time of deposition the marine 187 os 188 os ratios inferred from the mineoka umbers gradually increase up section with moderate fluctuations the 187 os 188 os composition ranging from 0 47 to 0 54 is significantly higher than that of the prior period post paleocene eocene thermal maximum petm 187 os 188 os 0 38 this was probably the result of an increase in the chemical weathering rate of continental crust caused by the long duration of noticeably warm climate during the early eocene greenhouse interval several fluctuations observed in the marine 187 os 188 os record during this interval are most likely related to petm like brief hyperthermals supporting the suggestion that the intensity of continental crustal weathering controlled by climatic changes can fluctuate on timescales of 10 5 yr

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

tracking millennial scale holocene glacial advance and retreat using osmium isotopes insights from the greenland ice sheet high resolution os isotope stratigraphy can aid in reconstructing pleistocene ice sheet fluctuation and elucidating the role of local and regional weathering fluxes on the marine os residence time this paper presents new os isotope data from ocean cores adjacent to the west greenland ice sheet that have excellent chronological controls cores msm 520 and da00 06 represent distal to proximal sites adjacent to two west greenland ice streams core msm 520 has a steadily decreasing os signal over the last 10 kyr 187os 188os 1 35 0 81 in contrast os isotopes from core da00 06 proximal to the calving front of jakobshavn isbrae highlight four stages of ice stream retreat and advance over the past 10 kyr 187os 188os 2 31 1 68 2 09 1 47 our high resolution chemostratigraphic records provide vital benchmarks for ice sheet modelers as we attempt to better constrain the future response of major ice sheets to climate change variations in os isotope composition from sediment and macro algae seaweed sourced from regional and global settings serve to emphasize the overwhelming effect weathering sources have on seawater os isotope composition further these findings demonstrate that the residence time of os is shorter than previous estimates of 104 yr

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

rhenium and osmium isotope and elemental behaviour accompanying laterite formation in the deccan region of india this study presents major trace element and re os isotope data for two laterite profiles from the deccan region in india the first profile from bidar developed upon deccan basalt itself the second from goa formed on an archean greywacke both profiles show the typical changes in mineralogy and chemistry accompanying laterite formation from little altered parent rock at depth to altered saprolite depleted in mobile elements to a highly altered si depleted fe rich laterite capped by an iron rich crust at the surface rhenium and os concentrations show a systematic enrichment with increasing degree of alteration where the highest concentrations are found in the fe rich laterite crust the level of os enrichment is greater than that of re and as a consequence 187re 188os ratios are lower in the laterite than the parent rock with time this leads to the development of 187os 188os ratios that are also less radiogenic than the parent rock and this relationship holds irrespective of rock type or age or the time of laterite formation the marked enrichment of both re and os and the patterns of change above and below the paleowatertable indicates that both elements are highly mobile high eh and low ph conditions facilitate re and os dissolution and redistribution occurs via groundwaters whereas with increasing ph higher in the profile both elements are scavenged out of solution and precipitate either as pge rich alloys or are strongly partitioned into oxides or oxyhydroxides nevertheless the isotope and elemental data for both profiles suggests that at least some of the re and os must be externally derived either from aeolian deposition or from the lateral movement of solute laden groundwaters given the high levels of os in laterites their extensive geographical coverage of the continents and their active role in the surface water cycle it seems likely that they will exert a significant influence on the continental flux of os to the oceans that will in turn be sensitive to tectonic or climatic change

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

the osmium isotopic composition of organic rich marine sediments abstract the osmium os concentration and 187 os 186 os ratio of several recent marine organic rich sediment samples from three widely separated sites have been measured os concentrations range from 0 095 to 0 212 ppb and 187 os 186 os ratios range from 8 2 to 8 9 the calculated fraction of hydrogenous os exceeds 78 in all samples thus the 187 os 186 os ratio of these samples reflects os isotopic composition of seawater the small range in measured 187 os 186 os ratio indicates that the os isotopic composition at these sites is fairly homogeneous the large magnitude of the os burial flux at these sites indicates the os burial in association with organic rich sediments is an important sink in the marine cycle of os these data also suggest that ancient organic rich sediments may provide a record of past variations in the os isotopic composition of 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

detrital sediment fluxes from continents to oceans abstract detrital sediment is carried from land to the sea by three agents rivers glaciers and winds the shoreline is an arbitrary boundary within the detrital sediment transport system which extends from a site of origin across areas of temporary storage to a site of long term deposition the most important of the agents moving sediment across the land is river transport estimated to be in the order of 20 10 12 kg of sediment annually at present analysis of drainage basins indicates that relief and runoff are the most important factors in determining the sediment load of rivers the competence of rivers to transport sediment is governed by the volume flow gradient and the sediment load itself today most large rivers are fed by snowmelt in highland areas runoff from rainfall in the drainage basin and groundwater inflow along the river course water is lost to evaporation and groundwater infiltration river courses can often be divided into two segments a degradational section in which the gradient is relatively steep and little temporary storage of sediment takes place and an aggradational section where the gradient is sharply reduced through meandering and where large scale temporary sediment storage forms a flood plain lakes trap sediment inland and prevent its transport to the sea today many high and mid latitude rivers are interrupted by lakes of glacial origin there are also some large areas of internal drainage that deliver no sediment to the sea the load carried by rivers has been markedly altered by human activity and may have doubled over the past few thousand years only to be reduced in the past century by the widespread construction of dams the ancient use of fire in hunting and its subsequent use in clearing land has increased erosion extensive deforestation and cultivation processes have also increased the sediment supply dam construction is a relatively new factor and affects the sediment transport system by trapping sediment before it can reach the sea the resulting lower sediment supply from rivers is at least in part compensated by increased coastal erosion glacial erosion is difficult to estimate there is an ongoing controversy whether ice sheets are effective erosive agents or not estimates of the present global flux of glacial detritus range from 0 8 50 10 12 kg annually with the lower value most probable the dust flux is in the order of 0 5 to 0 9 10 12 kg annually but may vary greatly with time

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

rhenium osmium and molybdenum isotope systematics of black shales from the lower cambrian niutitang formation sw china evidence of a well oxygenated ocean at ca 520 ma abstract the early cambrian was a key period in earth s history and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period new early cambrian data for re os and mo isotopic compositions and redox sensitive elements rse are reported here for the niutitang formation se chongqin sw china a re os age of 520 30 myr model 3 2 n 21 mswd 62 and a 187os 188os i ratio of 0 79 0 11 were obtained consistent with previously reported data for equivalent strata from other areas separate regression of samples with 187os 188os i of 0 72 0 79 and 0 81 0 89 produce more precise ages of 520 1 9 5 myr model1 2 n 7 mswd 1 0 and 513 10 myr model 1 2 n 8 mswd 0 96 respectively suggesting heterogeneity in the primary source of os as the dominant cause of scatter and uncertainty high initial 187os 188os of 0 79 0 11 may indicate a high os input from oxidative weathering of upper continental crust for early cambrian ocean based on rse data such as mo u v enrichment and ni co v cr v v ni moef uef re mo ratios two geochemically distinct zones upper and lower can be delineated in the sedimentary sequence as reflected in drillcore sections the upper section n 9 is characterized by low total organic carbon toc contents mean 1 8 1 5 wt low rse contents mo 0 008 0 006 10 3 g g v 0 22 0 15 10 3 g g u 0 008 0 004 10 3 g g and low v cr 2 0 1 7 ni co 5 9 3 5 moef uef 2 6 1 8 and mo toc 6 0 3 9 ratios in contrast the lower core section n 13 is characterized by high toc contents 4 4 2 2 wt high rse contents mo 0 10 0 08 10 3 g g v 1 0 1 0 10 3 g g u 0 05 0 04 10 3 g g high v cr 6 5 ni co 24 14 moef uef 6 3 2 2 and mo toc 21 9 ratios uncertainties are 1sd here significantly decreasing trends of those redox proxies from lower to upper sections suggest that the depositional conditions evolved from anoxic euxinic to oxic condition all samples display extremely low re mo ratios of 0 2 10 3 1 9 10 3 indicating the absence of intermediate reducing conditions possibly reflecting rapid ocean oxygenation the mo isotopic composition in the sedimentary succession of the yc9 core exhibits a large variation with 98 95mo values of 0 04 2 00 upper section 98 95mo values display an opposite trend to mn content and negative correlation with mo toc ratios indicating mo fractionation associated with adsorption on mn oxides or oxyhydroxides lower section 98 95mo values are positively correlated with mo toc ratios and negatively with re mo ratios suggesting incomplete conversion of molybdates to tetrathiomolybdates under weakly euxinic conditions high 98 95mo of 2 00 and high average mo toc ratios of 21 10 6 g g wt were observed in euxinic shales in the niutitang formation suggesting extensive ocean oxygenation may have taken place at ca 520 ma

major and trace elements of river borne material the congo basin the congo river basin is the second largest drainage basin in the world after the amazon the materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the african continent this paper presents the chemical composition of the different phases carried in the congo rivers and is followed by a companion paper devoted to the modelling of major and trace elements the congo river between bangui and brazzaville as well as its main tributaries including a few organic rich rivers also called black rivers were sampled during the 1989 high water stage the three main phases suspended load dissolved load and bedload were analysed for twenty five major and trace elements concentrations normalized to the upper continental crust show that in each river suspended sediments and dissolved load are chemical complements for the most soluble elements ca na sr k ba rb and u while these elements are enriched in the dissolved loads they are considerably depleted in the corresponding suspended sediments this is consistent with their high mobility during weathering another type of complementarity is observed for zr and hf between suspended sediments and bedload related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads compared to other rivers absolute dissolved concentrations of ca na sr k ba rb and u are remarkably low surprisingly high dissolved concentrations are found in the congo waters for other trace elements e g rees especially in the black rivers on a world scale these concentrations are among the highest measured in rivers and are shown to be ph dependent for a number of dissolved trace elements the dissolved loads are systematically normalized to the suspended loads for each river in order to remove the variations of the element abundances owing to source rock variations normalized diagrams for rees are presented and extended to the other elements they strongly support the argument that the apparent higher solubility of trace elements in the congo waters is due to the presence in the dissolved load of a colloidal phase as a result of 0 2 m filtration an important result is that these colloids are strongly depleted in fe and al with respect to the other elements finally the comparison of the dissolved suspended and sandy transport fluxes of each element in the congo basin rivers shows that although the proportions of for example the rees in the dissolved loads of the majority of the congo basin rivers is close to 10 of the total transport flux up to 80 of the rees are transported by the so called dissolved load in the black rivers

a global geochemical mass budget applied to the congo basin rivers erosion rates and continental crust composition rivers carry the products of continental denudation either in a dissolved form chemical erosion or in a solid form physical erosion we focus in this paper on the relationship between physical erosion and chemical erosion we establish the mass budget of the congo basin rivers using chemical complementarities between river suspended sediments sandy bedload and dissolved load of the congo basin rivers reported in a previous paper dupre et al 1995 a series of equations are presented assuming that the physical and chemical erosion processes are in a steady state during one year the total mass of river borne material dissolved and particulate transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time we show that the local continental crust on each drainage basin can be estimated and solve our steady state weathering model using an inversion procedure the very good agreement between modelled and measured values of the river suspended sediment concentrations validates the steady state hypothesis in this wet tropical area consequently in this area the sediment yield provide a good estimate of the rates of mechanical denudation this result also validates the calculation of the chemical and isotopic composition of the local continental upper crust using the bulk river load erosion rates for the silicate upper crust and thus independent of the lithological variability silicates evaporites and carbonates of the drainage basins are calculated mechanical erosion rates and chemical erosion rates for the congo basin at brazzaville are 8 t km2 y and 5 t km2 y the corresponding consumption of atmospheric co2 by weathering process is estimated to 51 103 mol km2 an these weathering and consumption rates are low in spite of the severity of the weathering conditions of the high soil temperature and of the intensity of precipitations these conclusions indicate the limiting influence the dynamic equilibrium of soils for silicate weathering finally by estimating the local continental crust chemical composition before the onset of erosion processes especially for the most soluble elements we can test the model of taylor and mclennan

deposition of osmium and other platinum group elements beneath the ultramafic hosted rainbow hydrothermal plume osmium and other platinum group elements pges have been measured in sediments accumulating directly under the rainbow hydrothermal plume at 36 deg n on the mid atlantic ridge 187os 188os and pge concentrations in the sediments of four cores taken 2 25 km from the active vent site reveal evidence of both radiogenic os scavenged from seawater and importantly unradiogenic os released by hydrothermal venting the majority of analyses n 16 yield 187os 188os ratios that fall in a narrow range 0 99 1 06 close to that of modern seawater in these samples os concentrations correlate linearly with hydrothermal v enrichments which are also derived from seawater scavenging a smaller subset of samples characterized by unusually high concentrations of ni and cr derived from ultramafic rock fragments exhibit distinctly lower 187os 188os ratios their ir content is also higher and their pge ratios are more similar to mantle material than the pge patterns of the larger set of samples dominated by seawater scavenged os although the two most proximal cores lack evidence for any similar ultramafic detritus in their surficial sediments 187os 188os ratios in these samples are also lower than that of ambient seawater suggesting they are likely to have been influenced by unradiogenic os from vent fluid incorporated into the plume samples from the most distal core by contrast exhibit 187os 188os ratios indistinguishable from that of seawater compared with background ne atlantic sediments os is enriched four fold in the seawater dominated samples while pd pt and ir are twice the background value burial fluxes of os scavenged by hydrothermal fe in these sediments are 200 400 pg cm 2 kyr 1 up to twice that found on the epr and several times the background bg of 50 pg cm 2 kyr 1 for pt burial fluxes are 1800 3000 pg cm 2 kyr 1 bg ca 1000 pg cm 2 kyr 1 and for pd 2700 4600 pg cm 2 kyr 1 bg ca 800 pg cm 2 kyr 1

accretion of extraterrestrial matter during the last 80 million years and its effect on the marine osmium isotope record the flux of particulate extraterrestrial et matter to the deep sea has been calculated using a four component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the pacific ocean nineteen published bulk leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation the calculated annual particulate flux averaged over several 100 ka by slow accumulation and bioturbation ranges from 18 000 to 67 000 tons with a mean value of 37 000 13 000 tons the data indicate no significant variability in the flux with time except at the k t boundary and are thus compatible with results based on ir accumulation in deep sea sediments the inferred constancy of the et matter flux is in contrast to recent results based on the et 3he flux that integrates only a small size fraction of the entire et matter flux 3he data indicate variability by a factor of 6 over the past 70 ma and a fivefold increase in the et flux between 3 ma and 1 ma that is not seen in the bulk et matter flux based on osmium isotope data the apparent constancy of the cosmic matter flux over the past 80 ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record except at the k t boundary a comparison between osmium isotope and ir data from two pelagic sediment cores from the pacific dsdp site 596 and ll44 gpc3 indicates that the seawater soluble fraction of et os cannot exceed 36 kg a and most likely is significantly smaller the maximum value of 36 kg a can account for approximately one half of the seawater soluble et matter flux necessary to balance the radiogenic continental runoff of os without any additional unradiogenic source simple mass balance calculations indicate that an additional unradiogenic source of os to the oceans most likely alteration of oceanic crust is required to balance the present day seawater osmium isotopic composition this source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater

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

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

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

provenance of dust in the pacific ocean eolian dust preserved in deep sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate in order to identify the provenance of eolian dust nd and sr isotopic compositions and rb sr and rare earth element ree concentrations have been determined for the silicate fractions of deep sea sediments from the north and central pacific ocean different regions of the pacific ocean are characterized by distinct air borne inputs producing a large range in end 10 to 1 stsr 86sr 0 705 0 721 la yb 5 15 eun eu 0 6 1 0 and sr nd 4 33 the average nd isotopic composition of pacific deep sea sediments eno 6 is more radiogenic than the average from the atlantic ena 8 in contrast the average 1475m 14and ratio for pacific sediments 0 114 is identical to that of atlantic sediments and to that of global average riverine suspended material the values of ena and 147sm 144nd are positively correlated for the pacific samples but negatively correlated for atlantic samples reflecting a fundamental difference between the dominant components in the end members with radiogenic nd island arc components in the pacific and lree enriched intraplate ocean island components in the atlantic samples from the north central pacific have distinctive unradiogenic end values of 10 87sr 86sr 0 715 high la yb 12 and low eun eu 0 6 and sr nd 3 6 these data are virtually identical to the values for loess from asia and endorse the use of these sediments as indicators of asian paleoclimate and paleowind directions island arc contributions appear to dominate in the northwest pacific resulting in higher end 1 to 1 and lower 87sr 86sr 0 705 and la yb 5 sediments from the eastern pacific tend to have intermediate sr and nd isotopic compositions but regionally variable sr nd and ree patterns they appear to be derived from the west margin of the north and south american continents rather than from asia our results confirm that dust provenance can be constrained by isotopic and geochemical analyses which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate

mechanical behavior of compacted composite clays composite clay is a mixture of clay as the main body and aggregates which are floating within the clayey matrix the undrained behavior of composite clay in its natural or compacted state e g

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

evaluating the shear strength of a natural heterogeneous soil using reconstituted mixtures this work originates from the need to provide the geotechnical characterisation of a natural heterogeneous soil outcropping along a landslide prone hillside the soil geologically identified as highly tectonised phyllite results from the large tectonic strain deformation of the original weak rock which produced a melange of grain particles enclosed in a fine matrix this study investigates the possibility of estimating the shear strength using the framework established for binary mixtures to overcome the difficulties of undisturbed sampling two main problems were tackled the identification of the matrix fraction in well graded mixtures and the effect of variation in the grading of the granular fraction

an osmium based method for assessing the source of dissolved rhenium and molybdenum to archean seawater abstract authigenic enrichments of re and mo in 2 7 2 5 ga organic rich mudrocks orm indicate that dissolved re and mo were present in late archean seawater however it can be difficult to determine if the dissolved re and mo were sourced from oxidative crustal weathering or magmatic hydrothermal extraterrestrial mhe inputs comparison of re mo and 192 os non radiogenic os isotope concentrations and initial 187 os 188 os isotope compositions from re os isochron regressions of orm can shed insight on the source of re and mo to archean seawater seawater 187 os 188 os reflects the relative contributions of radiogenic crustal versus unradiogenic mhe os inputs to seawater during the phanerozoic transient increases in magmatic hydrothermal activity marked by unradiogenic 187 os 188 os excursions and higher 192 os concentrations in orm are accompanied in some cases by higher re concentrations these unradiogenic os sources could thus have played a greater role in the archean marine re and possibly mo budget given that the riverine re and mo flux at that time was likely smaller than the phanerozoic flux hence a positive correlation between re mo and 192 os concentrations in archean orm that yields unradiogenic initial 187 os 188 os may reflect a mhe origin for dissolved re and mo in the overlying paleo seawater local oxidative weathering in the presence of photosynthetic o 2 of mafic ultramafic rocks and crustal sulfide minerals with low re os ratios is an alternative explanation but the two scenarios cannot be readily distinguished on the basis of initial 187 os 188 os by contrast an initial 187 os 188 os from archean orm that is statistically higher than the mhe baseline of 0 10 0 11 points to oxidative mobilization of re os and mo from crustal sulfide minerals rhenium and mo concentrations that are negatively correlated with 192 os concentrations in archean orm yielding unradiogenic initial 187 os 188 os suggests that some re and mo were derived from oxidative weathering such negative correlations may reflect an increased marine os sink adsorption to oxide minerals during a mild increase in local environmental o 2 levels these concepts may help trace the onset of photosynthetic o 2 production and accumulation along archean ocean margins

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

particle size distributions of some soils from the umbria region italy fractal analysis and numerical modelling abstract application of principles of fractal geometry has had a strong influence on the understanding of many geological processes one of the research fields which has benefited of the development of fractal methods is soil science in this field fractal geometry techniques mainly applied to the study of particle size distributions psd contributed to shed new light on the complex dynamics involved in soil forming processes in this contribution we present new data of psd of a number of soil samples from the umbria region italy particle size distributions are analysed by applying fractal geometry methods and results indicate that they display a fractal behaviour in addition two fractal scaling domains are identified characterizing different ranges of particle sizes to understand the basic mechanisms responsible for the production of fractal features observed in studied soil samples numerical models based on computer automata are presented it is shown that the numerical simulations are in good conceptual agreement with natural data and give the opportunity to explain the basic features observed in the psd of studied soil samples

great salinity anomalies in the north atlantic abstract we revisited the great salinity anomaly of the 1970s gsa 70s dickson et al 1988 and documented the newly identified great salinity anomaly of the 1980s hence termed gsa 80s both propagated around the north atlantic in a similar fashion the advective mechanism initially proposed to explain the observed sequence of low salinity low temperature events during the gsa 70s apparently holds also for the gsa 80s the latter was successively observed in the west greenland current 1982 the labrador current 1983 the flemish pass 1984 near the charlie gibbs fracture zone 1984 1985 in the rockall channel 1985 south of iceland 1985 1988 in the north sea 1986 1987 norwegian sea 1987 1988 barents sea 1988 1989 and iceland sea 1989 1990 the advection speed of the gsa 80s seems to be greater than the one of the gsa 70s the 1980s anomaly reached the barents sea 6 to 7 years after peaking in the west greenland current while the 1970s anomaly traveled the same route in 8 to 10 years these anomalies however seem to be of different origin the gsa 70s was apparently boosted remotely by a freshwater sea ice pulse from the arctic via fram strait consequently the gsa 70s was accompanied by a large sea ice extent anomaly in the greenland and iceland seas which propagated into the labrador sea in contrast the gsa 80s was likely formed locally in the labrador sea baffin bay mainly because of the extremely severe winters of the early 1980s but supplemented with a possible contribution of the arctic freshwater outflow via the canadian archipelago facilitated by strong northerly winds which would have enhanced stability and ice formation this anomaly was also associated with a positive sea ice extent anomaly in the labrador sea baffin bay which however had no upstream precursor in the greenland sea thus the gsas are not necessarily caused solely by an increased export of freshwater and sea ice from the arctic via fram strait these results are corroborated by the early 1990s data when a new fresh cold anomaly was formed in the labrador sea and accompanied by a large positive sea ice extent anomaly the harsh winters of the early 1990s were however confined to the labrador sea baffin bay area while the atmospheric and oceanic conditions in the greenland iceland and irminger seas were normal the labrador sea baffin bay area appears therefore to play a key role in formation of gsas as well as in propagation of the gsas formed upstream a likely contribution of the enhanced canadian archipelago freshwater outflow to the gsa formation also seems to be significant two major modes of the gsa origin are thus identified remote generated by an enhanced arctic ocean freshwater export via either fram strait or the canadian archipelago and local resulting from severe winters in the labrador sea baffin bay both modes should be taken into account to model decadal variability of the coupled ocean ice atmosphere system in the north atlantic arctic basin

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

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

os isotopes as tracers in seafloor hydrothermal systems metalliferous deposits from the tag hydrothermal area 26 n mid atlantic ridge abstract os isotopic analyses of a suite of samples from the tag hydrothermal area 26 n on the mid atlantic ridge reveal evidence of release of ocean crust derived os to seawater and removal of seawater derived os to the seafloor 187 os 186 os ratios of sulfide samples from the active tag mound vary between 8 7 and 1 3 the highest ratios are indistinguishable from the inferred 187 os 186 os ratio of seawater the incorporation of seawater derived os in these samples is interpreted as evidence of both entrainment of seawater into hydrothermal fluids and the addition of seawater os to samples during sulfide oxidation several sulfide samples from the active tag mound have 187 os 186 os ratios lower than the inferred seawater value with the lowest ratio approaching values expected for pristine oceanic crust these low 187 os 186 os ratios indicate that os derived from alteration of the oceanic crust is released to seawater in association with venting of high temperature hydrothermal fluids precipitates from low temperature hydrothermal fluids fe and mn oxide deposits from the east wall of the axial rift valley also have low 187 os 186 os ratios 4 2 6 8 relative to seawater these data suggest os isotopes may be sensitive tracers of low temperature hydrothermal circulation on the seafloor and that supply of ocean crust derived os to seawater is not strictly limited to high temperature hydrothermal vents the os concentration and isotopic composition of metalliferous sediment samples from a push core collected 125 m from the base of the active tag mound are also reported in this core metalliferous sediment horizons derived from mass wasting of altered sulfides exhibit variable 187 os 186 os ratios 4 8 8 7 reflecting mixing of seawater derived and ocean crust derived os in contrast sediments in which the metalliferous component is derived from fall out of hydrothermal material from the neutrally buoyant hydrothermal plume have 187 os 186 os ratios which vary over a narrow range 8 6 8 7 and appear to accurately record the os isotopic composition of seawater

osmium behavior in estuaries the lena river example the behavior of dissolved osmium at the river ocean interface was studied in the lena river estuary dissolved osmium removal is observed at very low salinities the loss is estimated to be 28 of the dissolved concentration of the river the removal cannot be related to the flocculation of iron oxide organic matter colloids but occurs simultaneously with the loss of aluminum the proposed mechanism is the adsorption of dissolved osmium on suspended particles in the maximum turbidity zone if this is correct then the removal of osmium in estuaries is probably a common phenomenon no contribution of osmium from the sediments is detected but neither the samples nor the stratified nature of the estuary are favorable to the study of a flux from sediments if this finding can be generalized the estimated global riverine flux of osmium to the ocean has to be recalculated and the osmium residence time in the ocean would change from 3 3 104 yr to 4 6 104 yr

re os isotope systematics in carbonates from serpent mound ohio implications for re os dating of crustal rocks and the osmium isotopic composition of ordovician seawater 1 eleven core samples of ordovician carbonates and shale from within and nearby the serpent mound ohio cryptoexplosion structure were analyzed for re os isotopes whole rock samples span a large range in measured 187os 188os 0 714 to 6 083 and exhibit a linear correlation of 187os 188os with 187re 188os the linear relationship is inconsistent with a meteorite crust mixing trend but could represent an errorchron with an age of 485 35 ma and an initial 187os 188os of 0 54 0 15 in order to assess whether the errorchron slope and initial 187os 188os ratio may provide constraints on depositional age and the 187os 188os ratio of ordovician seawater acetic acid and hcl leaching studies were done on selected samples the 87sr 86sr ratios of the leachates vary from 0 7081 to 0 7091 within the range found in middle upper ordovician brachiopods shields et al 2003 and consistent with the leachable fraction of the carbonates representing ordovician seawater compositions 187os 188os signatures for the leached samples are generally lower than those of respective whole rocks however five of six leachate samples produce a strong linear correlation on a re os isochron diagram the slope of which reflects an age of 443 34 ma consistent with biostratigraphic constraints on the depositional ages of the sampled rock units 445 472 ma the calculated initial 187os 188os of 0 53 0 04 is distinctly lower than present day seawater and if representative of ordovician seawater may reflect relatively low continental weathering rates due to waning pan african orogenies as proposed to explain the slow decrease in seawater 87sr 86sr starting in early ordovician qing et al 1998

characteristics and mass distribution of extraterrestrial dust from the greenland ice cap extraterrestrial grains with sizes 50 m can now be extracted in large numbers from terrestrial sediments but their origins are still uncertain at least 99 of these grains have been thought to be spherules1 2 resulting from the melting of their parent bodies in the atmosphere which destroyed their mineralogical composition here we present analyses of extraterrestrial grains extracted from six samples of cryoconite3 black dust collected from the melt zone of the greenland ice cap in addition to families of grains never reported before we have found a surprisingly high abundance of unmelted chondritic fragments in which the nonvolatile component of the parent bodies has been well preserved the mass distribution of the grains is very similar to that of the micrometeorite flux at 1 au ref 4 indicating that most of the grains are micrometeorites and not ablation products of larger meteorites these results yield new constraints on the ablation of such greenland micrometeorites as well as guidelines for using them as glaciological and geological markers

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

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

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

the 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

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