rapid stepwise onset of antarctic glaciation and deeper calcite compensation in the pacific ocean the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth at present this depth is 4 500 m with some variation between and within ocean basins the calcite compensation depth is linked to ocean acidity which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate1 geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the eocene oligocene boundary 34 million years ago 2 when significant permanent ice sheets first appeared on antarctica3 4 5 6 but the relationship between these two events is poorly understood here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical pacific ocean that cover the eocene oligocene boundary we find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40 000 years each synchronous with the stepwise onset of antarctic ice sheet growth the glaciation was initiated after climatic preconditioning7 by an interval when the earth s orbit of the sun favoured cool summers the changes in oxygen isotope composition across the eocene oligocene boundary are too large to be explained by antarctic ice sheet growth alone and must therefore also indicate contemporaneous global cooling and or northern hemisphere glaciation

reply to the comment by a g jones et al on deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and related quaternary volcanism 1 scientific discussion and different points of view are a basis of the advancement of knowledge we acknowledge the comments of jones et al 2012 as an opportunity to publicly discuss the structure and origin of the atlas mountains moreover we welcome the opportunity to compare our results with those recently published by the group responsible for the comment ledo et al 2011 although it is not pertinent to comment in detail on a paper published in another journal we also wish to remark that the paper of ledo et al 2011 was reviewed and published during the revision period of our contribution anahnah et al 2011 therefore they are two different approaches and data sets measured in different sites and by different instruments for the same region lending readers the chance to compare different interpretations the main differences on the data sets are the profile of anahnah et al 2011 compared with the profile of ledo et al 2011 is 170 km longer vertical magnetic data were obtained and lower frequencies were recorded 2 we regret the style and way used by jones et al 2012 we shall answer only those comments of jones et al 2012 related to objective issues 3 one of the final conclusions of jones et al 2012 might serve as the starting point of our reply

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

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

relationship between mass extinction and iridium across the cretaceous paleogene boundary in new jersey we directly link iridium ir anomalies in new jersey to the mass extinction of marine plankton marking the cretaceous paleogene k pg boundary we confirm previous reports of an ir anomaly 20 cm below the extinction of cretaceous macrofauna the pinna bed with new results from a muddy sand section from tighe park freehold new jersey united states but we also show that ir anomalies correlate with marine mass extinctions at three other clay rich new jersey sections thus we attribute the anomaly at freehold to the downward movement of ir and reaffirm the link between impact and mass extinction

os isotope chemostratigraphy applied to organic rich marine sediments from the eocene oligocene transition on the west african margin odp site 959 1 the known temporal relationship between the benthic foraminiferal 18o record and the marine os isotope record is used to reinterpret the absolute chronology and paleoceanographic context of an episode of organic carbon burial on the west african margin ocean drilling program odp site 959 although organic rich sediments require significant corrections for in situ decay of 187re to 187os these results demonstrate the utility of the marine os isotope record for chemostratigraphic correlation of organic rich sediments that are devoid of age diagnostic microfossils with pelagic carbonate sequences revision of the odp site 959 chronology shifts the age assignment of an interval of biosiliceous organic rich sediment deposition from the oligocene to the late eocene and earliest oligocene likely culminating with the first major glaciation of the oligocene oi1 we speculate that enhanced organic carbon burial over much of the west african margin may have contributed to drawdown of atmospheric carbon dioxide before and during the oi1 event and suggest that os isotope chemostratigraphy provides a valuable tool for further exploring this possibility

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

paleogene calcite compensation depth in the eastern subtropical pacific answers and questions 1 ocean drilling program leg 199 drilled a north south transect across the eocene paleoequator in the eastern pacific permitting reconstruction the calcite compensation depth ccd since earliest eocene time the ccd was relatively shallow near the early eocene pacific equator 3200 m and unlike modern latitudinal ccd gradients deepened to the north to 3600 m paleolatitude 10 n at 41 ma the ccd underwent a brief sharp transient deepening of 700 m then remained shallow until the eocene oligocene boundary at the e o boundary the ccd deepened by 1200 m in less than 300 kyr this rapid deepening served to more than double the area of seafloor subject to caco3 deposition sea level fall associated with ice volume buildup and ensuing shelf basin fractionation is unlikely to be the sole cause of the increased deep ocean caco3 burial rather a sudden rapid increase in the amount of ca entering the ocean appears necessary to explain the observations

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

radiogenic isotopic mapping of late cenozoic eolian and hemipelagic sediment distribution in the east central pacific pelagic clay of the east central pacific province is shown to be a mixture of three primary detrital components reflecting continental source areas in asia north america and central and south america relative contributions from each source area are a function of geography and this distribution appears to have remained constant over the past five million years despite changing flux rates a q mode factor analysis of downcore records for pb sr and nd isotopes identified three factors that account for 98 of the total variance these factors represent the radiogenic isotopic signatures of 1 late cenozoic asian dust which dominates in the central north pacific 2 north american continental hemipelagic eolian sources restricted mainly to the easternmost north pacific at 30 n latitude and 3 central and south american sources restricted to areas east of 100 w longitude south of the intertropical convergence zone 6 n the asian dust signature diminishes abruptly we conclude that late cenozoic asian dust sources can be isotopically differentiated downcore from both north american and south and central american sources in the eastcentral pacific this approach has a utility for identifying changes in long term cenozoic atmospheric circulation patterns

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 transport of osmium and strontium isotopes through a tropical estuary abstract we have determined the concentration and isotopic composition of os and sr in the estuarine waters from the godavari delta in peninsular india additionally we have obtained the concentration and isotopic composition of os and al concentration in selected suspended particulate matter recovered on 0 45 m filters the na k mg and ca concentrations of water samples obtained along salinity gradients from two distributary channels in the delta display a general two component mixing between river and sea water the data also reveal that al behaves non conservatively and is affected by interactions with suspended particulates the 87 sr 86 sr ratio of the riverine end member is 0 716303 and shows a linear decrease with salinity to seawater value and sr isotope systematics indicate that its behavior is conservative in the estuary the 187 os 188 os ratio of the godavari river end member is 1 24 and within error of the average eroding upper continental crust the concentration and isotopic composition of os through the two salinity transects shows that its behavior in the godavari estuary is complex and non conservative by comparing the al os ratios and os isotopes in the waters with those of the suspended particulate we find that both os gains and losses occur in the water column however in one of the distributaries vasishta the os concentration of suspended load increases and that of dissolved load decreases with increasing salinity towards the bay of bengal end member we infer that there is removal of seawater os at higher salinities the estimated mean residence time of os in the oceans is 37 14 2 kyr a comparison of the os concentration of the bay of bengal and the indian ocean waters indicates that the rainout rate of os in bay of bengal is 30 faster than that in the open ocean and suggests that the observed discrepancy between the mean residence time calculated from mass balance considerations and that estimated from the relaxation of the os isotopic ratio in marine record may not be real as the relaxation time experiments likely estimate the residence time for a basin sub basin and not for the entire ocean

the osmium isotopic composition change of cenozoic sea water as inferred from a deep sea core corrected for meteoritic contributions a detailed study was made of the oceanic 187os 186os variation through the cenozoic by using a hydrogen peroxide leaching procedure on a pelagic clay core from the north pacific long lines 44 gpc3 a 6 h2o2 solution was used the range of 187os 186os obtained by this procedure was between 2 and 8 2 a milder leach 0 15 h2o2 in three out of four samples yielded a slightly higher 187os 186os value than the stronger leach implying that the stronger leach released some meteoritic os with 187os 186os of 1 using published 3he concentrations in gpc3 and cosmic dust 3he and os fluxes to deep sea sediments the extraterrestrial os concentration was estimated for each core segment and subtracted to yield corrected values of 187os 186os the results based on the milder leaching protocol and the 3he correction protocol yield similar values the corrected ocean water 187os 186os pattern for the past 25 million years is similar to that obtained by other workers and is compatible with other results for the entire cenozoic the variation with time in gpc3 uniquely provides a statigraphic signature for the paleogene

geochemical assessment of the palaeoecology ontogeny morphotypic variability and palaeoceanographic utility of dentoglobigerina venezuelana to better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates a number of proxies are under development but some of the most promising e g palaeoseawater records of li and nd isotope change can only be employed on such large samples of mono specific foraminifera that application to the deep sea sediment core archive becomes highly problematic dentoglobigerina venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large 355 m diameter abundant and cosmopolitan planktic foraminifer that ranges from the early oligocene to early pliocene yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed here we assess the taxonomy palaeoecology and ontogeny of d venezuelana using stable isotope oxygen and carbon and mg ca data measured in tests of late oligocene to early miocene age from ocean drilling program odp site 925 on ceara rise in the western equatorial atlantic to help constrain the depth habitat of d venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within globigerina globigerinoides paragloborotalia and catapsydrax we define three morphotypes of d venezuelana based on the morphology of the final chamber and aperture architecture we determine the trace element and stable isotope composition of each morphotype for different size fractions to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat our data indicate that d venezuelana maintains a lower thermocline depth habitat at ceara rise between 24 and 21 ma comparing our results to published datasets we conclude that this lower thermocline depth ecology for the oligo miocene is part of an eocene to pliocene evolution of depth habitat from surface to sub thermocline for d venezuelana our size fraction data advocate the absence of photosymbionts in d venezuelana and suggest that juveniles calcify higher in the water column descending into slightly deeper water during the later stages of its life cycle our morphotype data show that 18o and 13c variation between morphotypes is no greater than within morphotype variability this finding will permit future pooling of morphotypes in the generation of the sample hungry palaeoceanographic records

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

tectonic setting origin and obduction history of the spontang ophiolite ladakh himalaya nw india abstract the spontang ophiolite forms the highest tectonic thrust slice above the mesozoic early tertiary continental margin of the north indian plate in the ladakh zanskar himalaya detailed field mapping combined with geochemical analysis has defined two major units a full ophiolite sequence spontang ophiolite overlain by an upper unit consisting of 500 m thick basalt andesite volcanic and volcano sedimentary rocks of island arc affinity spong arc the spontang ophiolite comprises a harzburgitic mantle sequence gabbroic and ultramafic cumulates isotropic gabbros and highly tectonized sheeted dikes feeding pillow lavas with a few uncommon highly fractionated plagiogranites a separate lherzolitic peridotite unit affected by possible transform related shearing was thrust over the harzburgites in the west possibly during the early stages of subduction initiation beneath the ophiolite whole rock geochemistry and u pb geochronology show that the ophiolite formed at a normal mid ocean ridge sp

high resolution 104 years deep sea foraminiferal stable isotope records of the eocene oligocene climate transition we have constructed high resolution 104 105 years benthic foraminiferal 13c and 18o records for the upper eocene through lower oligocene of two pelagic sequences deep sea drilling project dsdp site 522 in the angola basin south atlantic ocean and ocean drilling program odp site 744 in the southern indian ocean these records provide improved constraints on both the timing and magnitude of marine oxygen and carbon isotope events from 30 to 35 ma the oxygen isotope records indicate that the ubiquitous 18o increase oi 1 which marks the rapid expansion of continental ice sheets and a minimum of 3 to 4 c of cooling of bottom waters in the earliest oligocene 33 6 ma occurred in 350 kyr more than half the transition occurred over the final 40 50 kyr this period of lower temperatures and widespread continental glaciation persisted for roughly 400 kyr i e the duration of magnetochron c13n these records also indicate that this interval was characterized by at least two 100 kyr waxing and waning cycles oi 1a and oi 1b and possibly several higher frequency events the benthic foraminiferal 13c records show a positive 0 8 excursion that is nearly isochronous with the oi 1 oxygen isotope increase similar magnitude 13c increases at other sites indicate this was a global phenomenon suggestive of an unusually large perturbation to the carbon cycle this excursion was followed by smaller amplitude 13c oscillations with periods of roughly 400 kyr we suspect that the ubiquitous oi 1 13c excursion resulted from a brief but substantial increase in export production and carbon burial

early oligocene geomagnetic field behavior from deep sea drilling project site 522 hydraulic piston coring operations at deep sea drilling project site 522 in the south atlantic retrieved an unusually continuous section of late eocene to late oligocene pelagic sediments which we sampled at 3 4 cm intervals 3 5 kyr natural remanent magnetization demagnetization studies indicate a well behaved remanence various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo single domain magnetite appropriate for recording relative intensity variations of the paleomagnetic field nine zones of unusually low relative paleointensity were identified within the 2 my chron c12r interval seven can be typified by a 20 40 kyr interval of low field intensity accompanied by apparently random low amplitude short duration directional fluctuations the other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well defined virtual geomagnetic pole vgp paths confined along a preferred meridian of 70 90 w longitude we propose that both styles occur when the main dipole term diminishes significantly but that the former result when undiminished normal secular variation is continuous during the period of low axial dipole moment we propose that the other two lows in relative paleointensity along with one reversal record reflect a field structure of low axial dipole moment dominated by a low degree nonzonal spherical harmonic term alternatively the confined vgp paths could be an artifact of heavy remanence smoothing between nonantipodal semistable transitional geomagnetic pole positions geographical control of vgp paths particularly along 70 90 w longitude has recently been noted for much younger reversals the site 522 record may indicate that the underlying cause of this phenomenon was present at 32 ma we compare our c12r record of paleointensity lows with c12r marine magnetic anomaly tiny wiggles these data appear to indicate that c12r tiny wiggles resulted from periods of low geomagnetic field intensity that were sometimes accompanied by directional excursions

miocene seawater 187os 188os ratios inferred from metalliferous carbonates abstract seawater 187os 188os ratios for the middle miocene were reconstructed by measuring the 187os 188os ratios of metalliferous carbonates from the pacific dsdp 598 and atlantic dsdp 521 oceans atlantic and pacific 187os 188os measurements are nearly indistinguishable and are consistent with previously published os isotope records from pacific cores the atlantic data reported here provide the first direct evidence that the long term sedimentary 187os 188os record reflects whole ocean changes in the os isotopic composition of seawater the pacific and the atlantic os measurements confirm a long term 0 01 myr increase in marine 187os 188os ratios that began no later than 16 ma the beginning of the os isotopic increase coincided with a decrease in the rate of increase of marine 87sr 86sr ratios at 16 ma a large increase of 1 in benthic foraminiferal 18o values interpreted to reflect global cooling and ice sheet growth began approximately 1 million years later at 14 8 ma and the long term shift toward lower bulk carbonate 13c values began more than 2 myr later around 13 6 ma the post 16 ma increase in marine 187os 188os ratios was most likely forced by weathering of radiogenic materials either old sediments or sialic crust with a sedimentary protolith we consider two possible miocene specific geologic events that can account for both this increase in marine 187os 188os ratios and also nearly constant 87sr 86sr ratios 1 the first glacial erosion of sediment covered cratons in the northern hemisphere 2 the exhumation of the australian passive margin new guinea arc system the latter event offers a mechanism via enhanced availability of soluble ca and mg silicates in the arc terrane for the maintenance of assumed low co2 levels after 15 ma the temporal resolution three samples myr of the 187os 188os record from site 598 for which a stable isotope stratigraphy was also constructed is significantly higher than that of previously published records these high resolution data suggest oscillations with amplitudes of 0 01 to 0 02 and periods of around 1 myr although variations in the 187os 188os record of this magnitude can be easily resolved analytically this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent however the short term 187os 188os variations may correlate inversely with short term benthic foraminiferal 18o and bulk carbonate 13c variations that reflect glacioeustatic events

the marine osmium isotope record over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research several analytical developments particularly in the past few years have significantly increased our ability to study the behaviour of osmium in the surficial environment the 187os 188os and osmium concentration of seawater river water rain and hydrothermal vent fluids have been measured directly recently the behaviour of osmium in estuaries critical for estimating the marine residence time of osmium has been studied our knowledge of the surficial osmium cycle has thus significantly improved in addition reconstructions of past variations in the marine 187os 188os recently have been extended back into the mesozoic this review attempts to summarize our current understanding of the marine osmium system present and past the 187os 188os of seawater during the cenozoic to first order mimics the marine 87sr 86sr record it is therefore tempting to interpret both records as reflecting increased input of radiogenic osmium and strontium resulting from enhanced continental weathering regulated by climatic tectonic processes however the marine osmium isotope system differs fundamentally from the marine strontium isotope system this review emphasizes three important differences first large impacts are capable of resetting the 187os 188os to unradiogenic values without significantly affecting the marine strontium system second organic rich sediments are characterized by high 187re 188os resulting 187os 188os ingrowth trajectories are similar to the average slope of the cenozoic 187os 188os seawater record trends towards more radiogenic 187os 188os seawater therefore can be caused by weathering of organic rich sediments at a constant rate third the marine residence time of osmium is sufficiently short to capture short periodic glacial interglacial fluctuations that are inaccessible to the buffered marine strontium isotope system this offers the opportunity to discriminate between high frequency climatic and low frequency tectonic forcing

hydrochemical implications of groundwater mixing an example from the southern laramie basin wyoming the southern laramie basin groundwater system was studied to explore how standard thermodynamic chemical calculations can produce misleading results with respect to water rock interactions when groundwaters with different chemical compositions mix geological and hydrological data determined the direction of flow to be nw in the casper aquifers as well as in the redbeds aquifers hydrological and structural data were used to identify wells having mixtures of groundwater the chemical data revealed that waters in the different aquifers are distinguishable by their chemical compositions and that mixtures have intermediate compositions the different chemistries of the mixed groundwater samples are identical to those predicted by geochemical evolution paths including calcite precipitation evolution toward equilibration with gypsum and ca mg ion exchange

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

extraterrestrial ni rich spinel in upper eocene sediments from massignano italy evidence of a major cosmic event at the end of the eocene is given by the finding of at least one possibly two or more impact horizons containing microtektites microkrystites shocked quartz and unusually high iridium concentrations we report here the discovery in the global stratotype of the eocene oligocene boundary at massignano in italy of ni rich spinel a mineral that forms by the interaction of meteoritic bodies with the earth s atmosphere a maximum concentration of 800 crystals mg 1 corresponding to a flux of 2 107 crystals cm 2 is observed in a well dated layer 35 7 0 4 ma that also contains shocked quartz and iridium the crystals are found clustered in flattened spheroids the probable relic of their host bodies the chemical and physical characteristics of the spinel crystals indicate that the spheroids formed by interaction of dust particles in the upper part of the atmosphere rather than by ablation of large objects as proposed for the spinel bearing spheroids found at the cretaceous tertiary boundary the most likely explanation is that the dust particles were generated by a large cometary impact whether this impact is the one that produced microtektites and or microkrystites or another one that may have occurred shortly after or before is still questionable additional searching for ni rich spinel at other upper eocene sections may help to answer this question

controls over the strontium isotope composition of river water abstract strontium concentrations and isotope ratios have been measured in river and ground waters from the ganges orinoco and amazon river basins when compared with major element concentrations the data set has allowed a detailed examination of the controls over the strontium isotope systematics of riverine input to the oceans in the following environments 1 1 typical drainage basins containing limestones evaporites shales and alumino silicate metamorphic and igneous rocks 2 2 shield terrains containing no chemical or biogenic sediments and 3 3 the floodplains that constitute the largest areas of many large rivers the strontium concentration and isotope composition of river waters are largely defined by mixing of strontium derived from limestones and evaporites with strontium derived from silicate rocks the strontium isotope composition of the limestone endmember generally lies within the phanerozoic seawater range which buffers the 87 sr 86 sr ratios of major rivers a major exception is provided by the rivers draining the himalayas where widescale regional metamorphism appears to have led to an enrichment in limestones of radiogenic strontium derived from coexisting silicate rocks the strontium isotope systematics of rivers draining shield areas are controlled by the intense transportlimited nature of the weathering reactions and thereby limits variations in the strontium flux from these terrains floodplains are only a minor source of dissolved strontium to river waters and precipitation of soil salts in some floodplains can reduce the riverine flux of dissolved strontium to the oceans the most effective mechanisms for altering the isotope ratio and flux of riverine strontium to the oceans are increased glaciation and large scale regional metamorphism of the type produced during continental collision both mechanisms provide a means for increasing the 87 sr 86 sr ratio of the global riverine flux

geochemical evidence for a comet shower in the late eocene analyses of pelagic limestones indicate that the flux of extraterrestrial helium 3 to earth was increased for a 2 5 million year my period in the late eocene the enhancement began 1 my before and ended 1 5 my after the major impact events that produced the large popigai and chesapeake bay craters 36 million years ago the correlation between increased concentrations of helium 3 a tracer of fine grained interplanetary dust and large impacts indicates that the abundance of earth crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced these observations provide evidence for a comet shower triggered by an impulsive perturbation of the oort cloud

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

impact origin of the chesapeake bay structure and the source of the north american tektites seismic profiles drill core samples and gravity data suggest that a complex impact crater 35 5 million years old and 90 kilometers in diameter is buried beneath the lower chesapeake bay the breccia that fills the structure contains evidence of shock metamorphism including impact melt breccias and multiple sets of planar deformation features shock lamellae in quartz and feldspar the age of the crater and the composition of some breccia clasts are consistent with the chesapeake bay impact structure being the source of the north american tektites

combined osmium and strontium isotopic study of the cretaceous tertiary boundary at sumbar turkmenistan a test for an impact vs a volcanic hypothesis osmium and strontium isotopic ratios at the cretaceous tertiary boundary at sumbar turkmenistan display a negative hyperbolic covariation superimposed on the long term trend which displays a positive covariation the minimum ratio for os and the maximum ratio for sr occur at the boundary clay volcanism with a mantle or crustal source cannot account for the isotopic data the low 187 o s 188 o s and the high 87 sr 86 sr ratios can be explained by an impact whereby os was derived from the bolide and the 87 sr 86 sr ratio was enhanced by acid rain and or a tsunami following the event

shocked quartz from the late eocene impact evidence from massignano italy this paper reports the discovery of shocked quartz grains within a well dated upper eocene marlstone at the massignano outcrop ancona italy the shocked quartz reaches a maximum concentration of 1 6 0 4 grains per cubic centimetre of bulk rock a modest ir anomaly is also coincident with the shocked quartz and together they strongly support an impact origin moreover this impact layer has a precise interpolated radiometric age of 35 7 0 4 ma although impact evidence found throughout the world indicates that at least two large impacts did occur during this time the actual number timing and environmental effects of these impact events remain unclear the late eocene was a critical time in earth history because of significant climatic biotic and environmental crises

the accretion rate of extraterrestrial 3he based on oceanic 230th flux and the relation to os isotope variation over the past 200 000 years in an indian ocean core abstract in the eastern equatorial indian ocean the flux of extraterrestrial 3 he a proxy of interplanetary dust particles idps has been relatively constant over the past 200 ka the flux is equal to 1 1 0 4 10 12 cm 3 stp cm 2 ka 1 a value obtained using the xs 230 th profiling method variations in mass accumulation rates mars derived assuming a constant extraterrestrial 3 he flux have a 40 ka periodicity similar to that observed in the 18 o derived mars this frequency is similar to that of the earth s obliquity measured 187 os 188 os ratios are less radiogenic than present day seawater 0 49 0 98 reflecting the mixing of os derived from extraterrestrial terrigenous and hydrogenous sources when coupled with he data measured on the same samples os isotope data yield important information about the terrigenous component supplied to the eastern equatorial indian ocean the amount of os in the sample derived from the extraterrestrial component can be deduced with the help of the helium systematics once corrected for the extraterrestrial component of os os isotope signatures in conjunction with the 4 he concentrations suggest a supply of terrigenous material from indonesian ultramafic and himalayan crustal sources that clearly varies through time

improving confidence in ferromanganese crust age models a composite geochemical approach abstract accurate age models for marine ferromanganese fe mn crusts are essential to understand paleoceanographic changes and variations in local environmental factors affecting crust growth rate and their lateral continuity however no absolute method exists for dating these deposits beyond the age of 10 myr which requires the combination of a number of approaches here we present a composite age model for a 15 cm thick fe mn crust sample obtained by unique core drilling using a remotely operated vehicle at a water depth of 1130 m on the summit of tropic seamount in the north east atlantic the age model is based on cross validation of laser ablation u pb dating co chronometry and os isotopes these enable robust calibration of the age depth model using the bayesian statistical modelling of markov chain monte carlo mcmc simulations the results show that this fe mn crust commenced growth in the late cretaceous between 73 and 77 ma and grew at a rate between 1 and 24 mm myr averaging 4 mm myr the phosphatised carbonate substrate capping tropic seamount and underlying most of the fe mn crusts yields a u pb age of 84 4 myr and provides the upper age limit for the model less radiogenic excursions of 188os 187os in the vertical profile through the crust permit the identification of key inflection points in the os isotope seawater curve at the eocene oligocene and cretaceous paleogene transitions growth rates estimated from the empirical co chronometer are combined with the age envelope defined by the os data and used to validate the mcmc simulations the model identifies five hiatuses that occurred during the pliocene 2 5 1 9 5 3 1 7 ma early miocene 16 1 27 2 ma oligocene 29 2 32 1 ma eocene 41 2 52 0 6 ma and the late paleocene 55 1 59 1 4 ma a major phosphatisation event affecting the fe mn core can be dated to the late eocene 38 1 2 ma which coincides with a recorded change in the global oceanic system from warm and sluggish circulation to cold and vigorous thermohaline driven meridional overturn at the onset of antarctic glaciation

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

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

glacial weathering sulfide oxidation and global carbon cycle feedbacks connections between glaciation chemical weathering and the global carbon cycle could steer the evolution of global climate over geologic time but even the directionality of feedbacks in this system remain to be resolved here we assemble a compilation of hydrochemical data from glacierized catchments use this data to evaluate the dominant chemical reactions associated with glacial weathering and explore the implications for long term geochemical cycles weathering yields from catchments in our compilation are higher than the global average which results in part from higher runoff in glaciated catchments our analysis supports the theory that glacial weathering is characterized predominantly by weathering of trace sulfide and carbonate minerals to evaluate the effects of glacial weathering on atmospheric pco 2 we use a solute mixing model to predict the ratio of alkalinity to dissolved inorganic carbon dic generated by weathering reactions compared with nonglacial weathering glacial weathering is more likely to yield alkalinity dic ratios less than 1 suggesting that enhanced sulfide oxidation as a result of glaciation may act as a source of co 2 to the atmosphere back of the envelope calculations indicate that oxidative fluxes could change ocean atmosphere co 2 equilibrium by 25 ppm or more over 10 ky over longer timescales co 2 release could act as a negative feedback limiting progress of glaciation dependent on lithology and the concentration of atmospheric o 2 future work on glaciation weathering carbon cycle feedbacks should consider weathering of trace sulfide minerals in addition to silicate minerals

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

the marine 187 os 188 os record of the eocene oligocene transition the interplay of weathering and glaciation abstract osmium os isotope analyses of bulk sediments from the south atlantic equatorial pacific and the italian apennines yield a well dated and coherent pattern of 187os 188os variation from the late eocene to the early oligocene the resulting composite record demonstrates the global character of two prominent features of the low resolution ll44 gpc3 os isotope record pegram and turekian geochim cosmochim acta 63 1999 4053 4058 these are 1 a pronounced minimum in 187os 188os 0 22 0 27 in the late eocene between 34 and 34 5 ma and 2 a subsequent rapid increase in 187os 188os to approximately 0 6 by 32 ma an ultramafic weathering event and an increased influx of extraterrestrial particles to the earth are discussed as alternative explanations for the late eocene 187os 188os minimum comparison of the 187os 188os to benthic foraminiferal oxygen isotope records demonstrates that the nearly three fold increase in 187os 188os from the late eocene minimum coincides with the growth and decay of the first large ice sheet of the oligocene oi1 miller et al j geophys res 96 1991 6829 6848 the fine structure of the os isotope record indicates that enhanced release of radiogenic os unrelated to the recovery from late eocene minimum lagged the initiation of the oi1 event by roughly 0 5 myr this record in conjunction with weathering studies in modern glacial soils blum in w f ruddiman ed tectonic uplift and climate change plenum press new york 1997 pp 259 288 peucker ehrenbrink and blum geochim cosmochim acta 62 1998 3193 3203 suggests that exposure of freshly eroded material during deglaciation following oi1 enhanced chemical weathering rates and may have contributed to ice sheet stabilization by drawing down atmospheric carbon dioxide the improved temporal resolution and age control of the refined eocene oligocene os isotope record also makes it possible to illustrate the late eocene os isotope excursion as a tool for global correlation of marine sediments

reconstructing early himalayan tectonic evolution and paleogeography from tertiary foreland basin sedimentary rocks northern india the latest paleocene middle eocene subathu formation and the oligocene miocene dagshai and kasauli formations of the indian foreland basin record the early evolution of the himalayan fold thrust belt sandstone petrography of the subathu formation shows a predominantly recycled sedimentary source with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the dagshai formation sandstones in the dagshai and kasauli formations consist predominantly of metapelitic detritus the metamorphic grade of metapelitic lithic grains increases with time from dominantly very low grade at the base of the dagshai formation to dominantly low grade in the kasauli sandstones mudstone geochemistry documents the presence of a mafic ultramafic source during the time of deposition of the subathu formation that becomes significantly less important by the time of deposition of the dagshai formation compositions of subathu formation detrital spinels show they were either derived from both mid ocean ridge basalt type and arc type ophiolites or from an ophiolite of composite origin and that southerly derivation from the deccan trap continental flood basalts is unlikely kasauli formation garnet compositions suggest derivation from medium grade metamorphic rocks to amphibolite facies subathu formation composition reflects provenance influence from the indus suture zone during latest paleocene middle eocene time indicating initiation of continent continent collision and development of the foreland basin by this time suture zone influence was drastically reduced by the time of deposition of the dagshai formation when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the himalayan foreland basin the first appearance of himalayan metamorphic detritus occurs in the dagshai formation at the close of oligocene time whereas the kasauli formation records erosion to deeper metamorphic levels during earliest miocene time the occurrence of garnet and higher grade metamorphic lithic grains during early neogene time is coincident with the timing of displacement along the main central thrust and south tibetan detachment zone composition of early foreland basin sediments from pakistan balakot murree and kamlial formations to nepal bhainskati and dumri formations and bangladesh kopili barail formations surma group indicates diachronous arrival of ophiolitic to low grade metamorphic detritus derived respectively from the indus suture and early himalayan thrust sheets in the north this is consistent with progressively later closure of neotethys along the suture from latest paleocene time in the west to eocene time or even later in the east

evaluation of osmium isotopes and iridium as paleoflux tracers in pelagic carbonates new osmium os isotope and platinum group element pge concentration data are used in conjunction with published 3 he and th isotope data to determine the relative proportions of lithogenic extraterrestrial and hydrogenous iridium ir in a pacific pelagic carbonate sequence from the ocean drilling program odp site 806 on the ontong java plateau ojp these calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary ir budget are minor while hydrogenous ir accounts for roughly 85 of the total ir application of analogous partitioning calculations to previously reported data from a north pacific red clay sequence ll44gpc3 yields very similar results total ir burial fluxes at site 806 and ll44 gpc3 are also similar 45 and 30 pg cm 2 kyr 1 respectively average ir 3 he and ir xs 230 thinitial ratios calculated from the entire site 806 data set are similar to those reported earlier for pacific sites in general down core profiles of ir 3 he and xs 230 thinitial are not well correlated with one another however all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two while these results indicate that ir concentration has potential as a point paleoflux tracer in pelagic carbonates ir based paleoflux estimates are likely subject to uncertainties that are similar to those associated with co based paleoflux estimates consequently local calibration of ir flux in space and time will be required to fully assess the potential of ir as a point paleoflux tracer measured 187 os 188 os of the ojp sediments are systematically lower than the inferred 187 os 188 os of contemporaneous seawater and a clear glacial interglacial 187 os 188 os variation is lacking mixing calculations suggest os contributions from lithogenic sources are insufficient to explain the observed 187 os 188 os variations the difference between the 187 os 188 os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic os carried by particulate extraterrestrial material down core variations of 187 os 188 os with pt ir and os ir also point to contributions from extraterrestrial particles mixing calculations for each set of several triplicate analyses suggest that the unradiogenic os end member cannot be characterized by primary extraterrestrial particles of chondritic composition it is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions 187 os 188 os of pelagic carbonates has greater potential compared to abundances of pge an attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial os in the ojp samples and previously reported extraterrestrial os flux throughout most of the ojp record os isotope based paleoflux estimates are within a factor of two of those derived using other constant flux tracers meaningful flux estimates cannot be made during glacial maxima because the ojp sediments do not record the low 187 os 188 os reported previously we speculate that this discrepancy may be related to focusing of extraterrestrial particles at the ojp as has been suggested to explain down core 3 he variations

the tectonic evolution of the kohistan karakoram collision belt along the karakoram highway transect north pakistan the kohistan arc terrane comprises an intra oceanic island arc of cretaceous age separating the indian plate to the south from the karakoram asian plate to the north within the indus suture zone of north pakistan the intra oceanic arc volcanics chalt dras group were built on a foundation of dominantly mid ocean ridge basalt morb related amphibolites of the kamila group the subarc magma chamber is represented by multiple intrusions of a huge gabbro norite complex chilas complex which includes some ultramafic assemblages of residual mantle harzburgite and dunite layered cumulates and hornblendites cut by late stage dikes of hornblende plagioclase pegmatites the chilas complex norites intrude the gilgit metasediments of lower amphibolite and greenschist facies in northern kohistan which also form xenolithic roof pendants within the top of the chilas complex along the southern margin of kohistan jijal and sapat complex ultramafics dunites harzburgites and websterites form remnant suprasubduction zone ophiolitic mantle rocks along the hanging wall of the main mantle thrust the cretaceous obduction plane along which kohistan was emplaced onto indian plate rocks garnet granulites of the jijal complex formed at 12 14 kbars represent original magmatic lower crustal rocks subducted to depths of at least 45 km and metamorphosed during high pressure and high temperature subduction of earlier arc related rocks obduction of the sapat ophiolite and kohistan arc occurred between 75 and 55 ma the closure of the shyok suture zone separating kohistan from the karakoram plate must have occurred prior to 75 ma the age of the jutal basic dikes which crosscut the closure related fabrics mainly late north directed backthrusting in the lower hunza valley andean type granitoid gabbrodiorite granodiorite granite emplacement along the kohistan ladakh batholith ended at the time of india asia collision 60 50 myr ago postcollisional crustal thickening along the karakoram led to multiple episodes of metamorphism from latest cretaceous and throughout the tertiary sillimanite grade metamorphism in hunza was actually pre india asia collision and may have resulted from the earlier kohistan collision localized and sporadic crustal melting episodes across northern kohistan indus confluence and parri granite sheets and the southern karakoram hunza dikes and sumayar and mango gusar leucogranites occurred from 51 to 9 ma and culminated in the huge baltoro monzogranite leucogranite intrusion 25 21 myr ago a vast network of leucogranitic and pegmatitite dikes containing gem quality aquamarine muscovite tourmaline garnet biotite quartz are younger than 5 ma and form the final phase of intrusion in the haramosh area and parts of the southern karakoram area

early oligocene glaciation and productivity in the eastern equatorial pacific insights into global carbon cycling the onset of sustained antarctic glaciation across the eocene oligocene transition eot marks a pivotal change in earth s climate but our understanding of this event particularly the role of the

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

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

silicate weathering and carbon cycle controls on the oligocene miocene transition glaciation changes in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient mi 1 glaciation event at the oligocene miocene transition omt 23 ma however detailed geochemical proxy data are required to test these hypotheses here we present records of li ca mg ca cd ca u ca 18o 13c and shell weight in planktonic foraminifera from marine sediments spanning the omt in the equatorial atlantic ocean li ca values increase by 1 mol mol across this interval we interpret this to indicate a 20 increase in silicate weathering rates which would have lowered atmospheric co2 potentially forcing the antarctic glaciation circa 23 ma 13c of thermocline dwelling planktonic foraminifera track the global increase in seawater 13c across the omt and during the mi 1 event hence supporting a hypothesized global increase in organic carbon burial rates high 13c previously measured in epipelagic planktonic foraminifera and high cd ca ratios during mi 1 are interpreted to represent locally enhanced primary productivity stimulated by increased nutrients supply to surface waters the fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation inferred from high foraminiferal u ca and enhanced respiratory dissolution of carbonates characterised by reduced foraminiferal shell weight replication of our results elsewhere would strengthen the case that weathering induced co2 sequestration preconditioned climate for antarctic ice sheet growth across the omt and increased burial of organic carbon acted as a feedback that intensified cooling at this time

weathering flux and co2 consumption determined from palaeosol sequences across the eocene oligocene transition abstract rates of base cation depletion are determined for late eocene and early oligocene alluvial palaeosols pacific northwest of north america from pedogenic mass balance geochemistry and time estimates of palaeosol sequences pedogenic strain volume change and mass transfer of base cations silica and other pertinent elements are calculated from the geochemical differences between palaeosol horizons and parent material parent material compositions are determined by identification of the least altered alluvial strata in the sequence time spans of sequences of palaeosols are estimated from 40ar 39ar age dates of interbedded tuff beds and from time of formation estimates of palaeosols based on their degree of development palaeosol sequences from the mid latitude setting of central oregon document a significant decrease in the degree of weathering of individual palaeosols across the eocene oligocene transition as the climate became cooler and drier this decrease in the degree of weathering is accompanied by an increase in the accumulation rate of alluvial soils as indicated by sedimentation rates of stratigraphic packages according to these calculations and estimates the weathering flux of base cations in the john day basin measured in grams per 1000 years per square centimetre increased across the eocene oligocene transition these results when attributed to climate change indicate that weathering flux at this local increased as climate changed from warm humid late eocene conditions to cooler drier oligocene conditions furthermore the extrapolation of these weathering flux results to other mid latitude settings with similar climate change history may help to explain the severe climate change of the eocene oligocene boundary it is proposed that in some mid latitude settings greater quantities of less weathered silicate material were exposed during the early oligocene than during the late eocene thus causing an increase in flux of weathered product increased weathering flux during this time in certain regions of the mid latitudes may have increased both co2 consumption and bicarbonate transport to the ocean and contributed to atmospheric pco2 drawdown and global cooling

cenozoic deep sea temperatures and global ice volumes from mg ca in benthic foraminiferal calcite a deep sea temperature record for the past 50 million years has been produced from the magnesium calcium ratio mg ca in benthic foraminiferal calcite the record is strikingly similar in form to the corresponding benthic oxygen isotope 18o record and defines an overall cooling of about 12 c in the deep oceans with four main cooling periods used in conjunction with the benthic 18o record the magnesium temperature record indicates that the first major accumulation of antarctic ice occurred rapidly in the earliest oligocene 34 million years ago and was not accompanied by a decrease in deep sea temperatures

early cenozoic glaciation antarctic weathering and seawater 87sr 86sr is there a link abstract stable and radiogenic isotopic and sedimentological data from sub antarctic deep sea sediment cores reveal a temporal link between changes in seawater 87 sr 86 sr ratios and major episodes of late eocene early oligocene climate change the 87 sr 86 sr records show two major inflections one at 38 39 ma near the middle late eocene boundary followed by another at 33 4 ma similarly the oxygen isotope ice rafted debris and clay assemblage records indicate two important climatic events the appearance of alpine glaciers and or small ice sheets on antarctica in the late eocene at 38 39 ma followed by a rapid transition to larger and more permanent temperate ice sheets in the early oligocene at 33 4 ma moreover during the early oligocene 30 33 ma three to four inferred peaks in glacial activity appear to coincide with subtle steps in the 87 sr 86 sr record the coupled variations in climate and seawater sr isotope ratios during the eocene oligocene imply a strong causal link between the two either changes in climate directly influenced patterns of continental weathering and hence seawater chemistry and or a tectonic event e g uplift as reflected in weathering and seawater chemistry triggered relatively abrupt changes in global climate

the age of the popigai impact event and its relation to events at the eocene oligocene boundary ages ranging from the late cretaceous 65 myr to the oligocene 29 myr have been reported for the 100 km diameter popigai impact structure1 2 on the anabar shield central siberia these ages3 4 5 6 7 8 overlap the timing of several possible impact related features including the cretaceous tertiary and eocene oligocene stratigraphic boundaries the north american tektites9 and the recently reported occurrences of an iridium anomaly and shocked quartz in late eocene deposits in northern italy10 here we report age determinations of several popigai impact melt rocks using the 40ar 39ar step heating technique to constrain the age of the impact event our results are consistent with a late eocene impact age of 35 7 0 2 myr 2 coincident in time with the impact deposits found in italy as this age is also similar to that of the north american tektites which have been associated with the chesapeake bay impact structure in the eastern united states11 12 13 there seem to have been at least two large and essentially contemporaneous impacts during the late eocene