the geohydrologic setting of yucca mountain nevada abstract this paper provides a geologic and hydrologic framework of the yucca mountain region for the geochemical papers in this volume the regional geologic units which range in age from late precambrian through holocene are briefly described yucca mountain is composed of dominantly pyroclastic units that range in age from 11 4 to 15 2 ma the principal focus of study has been on the paintbrush group which includes two major zoned and welded ash flow tuffs separated by an important hydrogeologic unit referred to as the paintbrush non welded ptn the regional structural setting is currently one of extension and the major local tectonic domains are presented together with a tectonic model that is consistent with the known structures at yucca mountain streamflow in this arid to semi arid region occurs principally in intermittent or ephemeral channels near yucca mountain the channels of fortymile wash and amargosa river collect infrequent runoff from tributary basins ultimately draining to death valley beneath the surface large scale interbasin flow of groundwater from one valley to another occurs commonly in the region regional groundwater flow beneath yucca mountain originates in the high mesas to the north and returns to the surface either in southern amargosa desert or in death valley where it is consumed by evapotranspiration the water table is very deep beneath the upland areas such as yucca mountain where it is 500 750 m below the land surface providing a large thickness of unsaturated rocks that are potentially suitable to host a nuclear waste repository the nature of unsaturated flow processes which are important for assessing radionuclide migration are inferred mainly from hydrochemical or isotopic evidence from pneumatic tests of the fracture systems and from the results of in situ experiments water seeping down through the unsaturated zone flows rapidly through fractures and more slowly through the pores of the rock matrix although capillary forces are expected to divert much of the flow around repository openings some may drip onto waste packages ultimately causing release of radionuclides followed by transport down to the water table

sea surface temperature history of the low latitude western pacific during the last 5 3 million years abstract the late neogene was a time of cryosphere development in the northern hemisphere the present study was carried out to estimate the sea surface temperature sst change during this period based on the quantitative planktonic foraminiferal data of 8 dsdp sites in the western pacific target factor analysis has been applied to the conventional transfer function approach to overcome the no analog conditions caused by evolutionary faunal changes by applying this technique through a combination of time slice and time series studies the sst history of the last 5 3 ma has been reconstructed for the low latitude western pacific although the present data set is close to the statistical limits of factor analysis the clear presence of sensible variations in individual sst time series suggests the feasibility and reliability of this method in paleoceanographic studies the estimated sst curves display the general trend of the temperature fluctuations and reveal three major cool periods in the late neogene i e the early pliocene 4 7 3 5 ma the late pliocene 3 1 2 7 ma and the latest pliocene to early pleistocene 2 2 1 0 ma cool events are reflected in the increase of seasonality and meridional sst gradient in the subtropical area the latest pliocene to early pleistocene cooling is most important in the late neogene climatic evolution it differs from the previous cool events in its irreversible steplike change in sst which established the glacial climate characteristic of the late pleistocene the winter and summer sst decreased by 3 3 5 4 c and 1 0 2 1 c in the subtropics by 0 9 c and 0 6 c in the equatorial region and showed little or no cooling in the tropics moreover this cooling event occurred as a gradual sst decrease during 2 2 1 0 ma at the warmer subtropical sites while that at cooler subtropical site was an abrupt sst drop at 2 2 ma in contrast equatorial and tropical western pacific experienced only minor sst change in the entire late neogene in general subtropics was much more sensitive to climatic forcing than tropics and the cooling events were most extensive in the cooler subtropics the early pliocene cool periods can be correlated to the antarctic ice volume fluctuation and the latest pliocene early pleistocene cooling reflects the climatic evolution during the cryosphere development of the northern hemisphere

south china sea surface water evolution over the last 12 myr a south north comparison from ocean drilling program sites 1143 and 1146 1 planktonic foraminifera pf from ocean drilling program odp sites 1143 and 1146 in the southern and northern south china sea scs respectively were quantitatively analyzed in order to reconstruct the sea surface environment over the last 12 myr the observed decrease in deep dwelling pf species after 10 ma at both sites is interpreted to reflect a depression of the upper water thermocline corresponding to the closure of the indonesian seaway around 11 9 ma this upper water column structure implies the intensification of equatorial pacific warm currents and the initial formation of the western pacific warm pool wpwp during the early late miocene the consistent pattern of south north thermocline evolution and the synchronous disappearance of globoquadrina dehiscens 9 8 ma at both sites 1143 and 1146 together imply that the entire scs was likely under the influence of the newly developed wpwp at 10 ma after 8 ma sea surface temperatures and thermocline variations evolved differently between the southern and northern scs the total deep dwelling pf fauna at site 1143 decreased gradually in abundance from 6 6 to 2 ma indicating a deepening of the thermocline in the southern scs in contrast deep dwelling pf species increased in abundance from 3 1 to 2 ma at site 1146 reflecting a shoaling of the thermocline in the northern scs this south north contrast reflects two major environmental regimes 1 the southern scs which has mainly been under the influence of the wpwp since the late late miocene and 2 the northern scs where effects of the east asian winter monsoon have prevailed especially since the late pliocene estimate of past sea surface temperatures ssts at site 1143 suggests a relatively stable and warm environment in the southern scs since about 2 5 ma with an increased influence of warm subsurface waters after the mid pleistocene transition 1 2 0 9 ma in the northern scs however a gradual decrease in winter sst recorded at site 1146 over the last 4 myr records east asian monsoon evolution especially the enhancement of the east asian winter monsoon between 3 1 and 2 ma

alteration of uraninite from the nopal i deposit pen a blanca district chihuahua mexico compared to degradation of spent nuclear fuel in the proposed u s high level nuclear waste repository at yucca mountain nevada abstract at the nopal i uranium deposit primary uraninite nominally uo 2 x has altered almost completely to a suite of secondary uranyl minerals the deposit is located in a basin and range horst composed of welded silicic tuff uranium mineralization presently occurs in a chemically oxidizing and hydrologically unsaturated zone of the structural block these characteristics are similar to those of the proposed u s high level nuclear waste hlw repository at yucca mountain nevada petrographic analyses indicate that residual nopal i uraninite is fine grained 5 10 m and has a low trace element content average about 3 wt these characteristics compare well with spent nuclear fuel the oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the nopal i system e g si ca k na and h 2 o and also common to yucca mountain in contrast secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the yucca mountain environment e g pb p and v the oxidation of nopal i uraninite and the sequence of alteration products their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated uo 2 under conditions intended to approximate those anticipated for the proposed yucca mountain repository the end products of these reported laboratory experiments and the natural alteration of nopal i uraninite are dominated by uranophane nominally ca uo 2 2 si 2 o 7 6h 2 o with lesser amounts of soddyite nominally uo 2 2 sio 4 2h 2 o and other uranyl minerals these similarities in reaction product occurrence developed despite the differences in time and physical chemical environment between yucca mountain approximate laboratory experiments and yucca mountain approximate uraninite alteration at nopal i suggesting that the results may reasonably represent phases likely to form during long term alteration of spent fuel in a yucca mountain repository from this analogy it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the yucca mountain environment may be relatively limited and may be insensitive to small variations in system conditions

response of upper ocean structure to the initiation of the north hemisphere glaciation in the south china sea abstract planktonic foraminifer as well as oxygen and carbon isotopic records from ocean drilling project core 1146 located at the northern continental slope of the south china sea were used to study the response of the upper water column structure to the formation and progressive intensification of the northern hemisphere glaciation during the late pliocene 3 2 2 0 ma variations in the relative abundance of the globigerinoides ruber globigerinoides sacculifer globorotalia menardii and globorotalia inflata groups globorotalia crassaformis and neogloboquadrina pachyderma showed that sea surface temperatures gradually decreased coinciding with heavier oxygen isotope values at 2 8 2 72 2 6 2 5 2 16 and 2 08 ma after 2 7 ma the relative abundance of mixed layer species decreased while that of thermocline dwellers dominated by high productivity species increased 13 c variations among g sacculifer pulleniatina obliquiloculata and cibicidoides wuellerstorfi suggest that the mixed layer depth and vertical exchange was enhanced with the strenghtening of the east asian winter monsoon which in turn is associated with the progressive intensification of the northern hemisphere glaciation

methodology for bounding calculations of nuclear criticality of fissile material accumulations external to a waste container at yucca mountain nevada abstract the possibility of nuclear criticality however remote in the vicinity of the proposed repository at yucca mountain nevada generates justified concerns and may impact the performance of the repository a heuristic approach is presented here for determining the amount spatial distribution and other characteristics of fissile material accumulation in the rock beneath a waste package that could contribute to such an event this study is concerned primarily with waste packages containing special spent fuel from the department of energy and high level nuclear waste glass mixing with less alkaline waters and the subsequent drop in ph is the mechanism that is most efficient for precipitating fissile material from the waste package internal leachate in contrast to natural deposits in which redox changes are the main precipitation driver external accumulation size is determined by 1 computing the chemical composition of the leachate leaving a package as its internal materials degrade with the batch geochemical code eq3 6 2 determining precipitation of fissile material into mineral phases using the 1d geochemical code phreeqc as the effluent mixes with percolation water and 3 heuristically scaling results to a 3d volume and computing the criticality coefficient using the code mcnp loci for accumulation are the multiple lithophysal cavities and the fracture system a bounding conservative approach is used by necessity in step 3 nuclear criticality is sensitive to small variations in the distribution of fissile material and parameters of natural systems vary by orders of magnitude because the most likely combinations of parameters are not conducive to nuclear criticality this study focuses on extreme values of parameter probabilistic distributions such as limited flow into the package associated with a large percolation rate combinations of material degradation rates favoring actinide release and very high host rock porosity values by considering these combinations most favorable to criticality but unlikely it was concluded that external nuclear criticality is not a concern at the proposed repository

percolation flux and transport velocity in the unsaturated zone yucca mountain nevada abstract the percolation flux for borehole usw uz 14 was calculated from 14 c residence times of pore water and water content of cores measured in the laboratory transport velocity is calculated from the depth interval between two points divided by the difference in 14 c residence times two methods were used to calculate the flux and velocity the first method uses the 14 c data and cumulative water content data directly in the incremental intervals in the paintbrush nonwelded unit and the topopah spring welded unit the second method uses the regression relation for 14 c data and cumulative water content data for the entire paintbrush nonwelded unit and the topopah spring tuff topopah spring welded unit using the first method for the paintbrush nonwelded unit in borehole usw uz 14 percolation flux ranges from 2 3 to 41 0 mm a transport velocity ranges from 1 2 to 40 6 cm a for the topopah spring welded unit percolation flux ranges from 0 9 to 5 8 mm a in the 8 incremental intervals calculated transport velocity ranges from 1 4 to 7 3 cm a in the 8 incremental intervals using the second method average percolation flux in the paintbrush nonwelded unit for 6 boreholes ranges from 0 9 to 4 0 mm a at the 95 confidence level average transport velocity ranges from 0 6 to 2 6 cm a for the topopah spring welded unit and topopah spring tuff average percolation flux in 5 boreholes ranges from 1 3 to 3 2 mm a average transport velocity ranges from 1 6 to 4 0 cm a both the average percolation flux and average transport velocity in the ptn are smaller than in the ts tsw however the average minimum and average maximum values for the percolation flux in the ts tsw are within the ptn average range therefore differences in the percolation flux in the two units are not significant on the other hand average average minimum and average maximum transport velocities in the ts tsw unit are all larger than the ptn values implying a larger transport velocity for the ts tsw although there is a small overlap

pliocene pleistocene stable isotope and paleoceanographic changes in the northern south china sea abstract based on the stable isotopic analysis of planktonic and benthic foraminifers from ocean drilling program core 1148 of the northern south china sea scs pliocene pleistocene isotope stratigraphy and events have been reconstructed the benthic foraminiferal 18o record shows that the pacific intermediate water had a greater influence upon the scs or the pacific deep water above 2600 m was warmer before 3 2 ma than at present after that the benthic 18o conspicuously increased during the 3 2 2 5 ma period in correspondence to the formation of the northern hemisphere ice sheet whereas the planktonic 18o signal suggests a stepwise overall decrease of sea surface temperature during the 2 2 0 9 ma period compared to the equatorial pacific records the decrease in planktonic globigerinoides ruber 13c during the 3 2 2 2 ma period is particularly striking suggesting that fertility of surface water increased noticeably according to the modern 13c distribution of g ruber in the northern scs it is inferred that the east asian winter monsoon strengthened during this interval afterwards there were several conspicuous decreases of g ruber 13c at 1 7 1 3 0 9 0 45 and 0 15 ma bp that is about every 0 4 ma suggesting that the east asian winter monsoon became episodically stronger this is confirmed by changes in relative abundance of planktonic foraminifer species neogloboquadrina dutertrei a typical east asian winter monsoon proxy the deep water 13c of the scs is close to that of the pacific but lighter than that of the atlantic implying that the pattern of deep water originating mainly from the atlantic and through the pacific entering the scs existed at least since the early pliocene after 1 4 ma the benthic 13c signal decreased conspicuously but with a periodicity of 100 ka suggesting that the deep water ventilation of the scs was reduced probably corresponding to a decrease of the north atlantic deep water and or further isolation of the scs deep basin from the pacific during glaciations

recent planktonic foraminifers and their relationships to surface ocean hydrography of the south china sea the relative abundances of recent planktonic foraminifers from surface sediments on the sea floor reveal relationships to hydrographic variables of the ocean surface to investigate whether the evect of the vertical structure of oceanic upper layer environments is important to the changes in the abundance of planktonic foraminifers a set of 173 coretop faunal data from modern south china sea scs surface sediments was compiled for comparison with direct observations of sea surface temperature sst and the depth of thermocline dot the coretops used in this study are distributed within the area of 25 n and 105 125 e with water depths ranging from 68 to 3990 m the relationships between the abundances of planktonic foraminifer species and hydrographic variables were examined using simple correlation analyses in the analyses four groups of planktonic foraminifers with diverent ecological preferences were identified group i faunas primarily reliant on dot with positive correlations n dutertrei group ii faunas reliant on both sst and dot showing negative correlations with sst and positive correlations with dot g glutinata p obliquiloculata and g bulloides group iii faunas reliant on both sst and dot showing positive correlations with sst and negative correlations with dot including g sacculifer g menardii and g aequilateralis and group iv faunas reliant on both sst and dot with positive correlations g ruber though each of these species displays selectivity of ecological controls these results indicate that in the scs correlations between faunal abundances of planktonic foraminifers and sst or dot are about equally significant this study also indicates a need for re evaluating the relationships between planktonic foraminifer abundances and many other important hydrographic variables in the upper layer of oceans on a more regional scale 1998 elsevier science b v all rights reserved

vertical distribution and isotopic fractionation of living planktonic foraminifera from the panama basin a model1 2 which explains the vertical and seasonal distribution of planktonic foraminifera was developed using data from the western north atlantic and is comprised of three tenets 1 the conditions in the photic zone upper 80 100 m exclusively dictate planktonic foraminifera species composition and abundance throughout the water column 2 species are vertically stratified within the photic zone according to their temperature preference depth distribution according to temperature preference has been proposed by several authors3 based on isotope analyses of sediments due to seasonal temperature variations seasonal variations in the 18o composition of seawater and species specific isotope fractionation rigorous proof of this hypothesis cannot be provided from sediment analyses alone 3 the chlorophyll maximum is a major food source zone which is preferentially exploited by foraminifera our data from the panama basin which are presented here are confirmation of this model the panama basin in the eastern equatorial pacific which has a shallow and steep thermocline was sampled for living planktonic foraminifera a multiple opening closing net and environmental sensing system mocness 4 was used to collect 13 sets of eight vertically stratified oblique samples we report here the quantitative distribution and stable isotopic composition of planktonic foraminifera from four mocness tows taken over 5 days during august 1979 rv knorr cruise 73 at 5 20 n 8 50 w each tow consisted of eight samples which integrated 12 5 m intervals from 0 to 100 m for moc 126 25 m intervals from 0 to 200 m for moc 127 100 150 m intervals from 0 to 1 000 m on moc 128 and 250 m intervals from 0 to 2 000 m on moc 131

faunal distribution patterns of planktonic foraminifers in surface sediments of the low latitude pacific abstract faunal indices of sea surface temperature sst have been widely used for reconstructing past ocean variability and for verifying the results of climate modeling recent studies on the distribution patterns of modern planktonic foraminifers on surface sediments have indicated important relationships with many hydrographic variables associated with the vertical structure of upper layer oceans to investigate whether these other ocean hydrographic variables are important to the changes in the faunal distribution pattern two sets of core top faunal data from modern low latitude pacific surface sediments were compiled to compare with direct observations of sst as well as the depth of thermocline dot a primarily important variable for describing the vertical structure of upper layer oceans at low latitudes the degree of correlation between the faunal principal components defined by a q mode principal component analysis sst dot and a carbonate preservation index cpi was examined using simple and partial correlation analyses this study indicates that the effect of dot controls on faunal distributions of planktonic foraminifers in the low latitude pacific is important and interpreting the dot or other hydrographic variables of upper layer oceans from planktonic foraminifer faunal data would be more appropriate in future paleoceanographic applications

late quaternary upwelling intensity and east asian monsoon forcing in the south china sea foraminifera from two cores off eastern vietnam and the northwestern philippines where modern summer and winter monsoon driven upwelling occurs in the south china sea respectively were analyzed to evaluate the changes in paleoproductivity and upper water structure over the last 220 000 yr we observed enhanced organic carbon flux and a shoaled thermocline when upwelling intensified off eastern vietnam during interglacial ages and off the northwestern philippines during glacial ages this indicates that the east asian summer monsoon increased while the winter monsoon decreased during interglacial ages particularly the upwelling reached a maximum off eastern vietnam during late marine isotopic stage mis 5 and off the northwestern philippines during mis 2 implying that the summer monsoon decreased gradually since mis 5 while the winter monsoon displayed an opposite trend the variations in upwelling proxies exhibit a distinct cyclicity with frequencies near 41 000 yr and 23 000 yr off eastern vietnam in contrast to a strong frequency peak near 100 000 yr off the northwestern philippines we suggest that the east asian summer monsoon has been forced by changes in solar insolation associated with precession and obliquity while ice volume forcing is probably a primary factor in determining the strength and timing of the east asian winter monsoon but with less important insolation forcing

pliocene pleistocene evolution of eastern tropical pacific surface water circulation and thermocline depth we evaluated the response of eastern equatorial pacific upper water column hydrography to the panama seaway closure and the initiation of large scale northern hemisphere glaciation the 18o gradient between globorotalia tumida and globigerinoides sacculifer indicates a general shoaling of the thermocline between 4 2 and 3 0 ma which may be related to a recorded decrease in southeast trade wind strength the 13c gradients suggest that surface water nutrient concentrations have increased during the last 5 myr and that upwelling intermediate waters began to change sources at 3 2 ma changes in the trans pacific east west temperature and nutrient gradients at 4 0 and 1 5 ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation changes in the sensitivity of surface circulation to milankovitch forcing 18o records linearly related to orbital variations only after 3 2 ma are associated with the seaway closure pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to milankovitch forcing is enhanced by air sea interactions which maintain the relatively steep pleistocene trans pacific gradients the 13c records are dominated by the 41 kyr period and are usually coherent with high latitude climate suggesting that associated changes in the global carbon reservoir dominate the tropical 13c signals

uranium transport through fractured silicic tuff and relative retention in areas with distinct fracture characteristics abstract the nopal i uranium u deposit in the pefia blanca district chihuahua mexico has been identified as analogous in some regards to the candidate u s high level waste hlw repository at yucca mountain nevada uranium transport at the nopal i deposit has been studied to investigate mechanisms by which hlw components could be transported through silicic tuff over long time periods this investigation focused on approximately 1400 m2 of essentially continuous bedrock outcrop spanning the nopal i deposit and surrounding host tuff data collected document i the distributions of u within and around the nopal i deposit ii the distribution and characteristics of the fracture network within and surrounding the deposit and iii the transport of u away from the deposit mainly along fracture paths uranium series isotopic measurements indicate mobilization of u along the margin of the deposit within the last 1 ma and significant u transport at about 54 ka transport of u away from the nopal i deposit along a few relatively continuous mesofractures achieved maximum distances at least 20 times greater than transport through the general fracture network composed of thousands of less continuous microfractures within and surrounding the deposit uranium transport away from the deposit appears to be largely independent of variations in the general fracture network pattern transport of u away from individual micro and meso fractures into homogeneous unfractured tuff matrix appears limited to distances less than 1 mm at the nopal i deposit matrix diffusion does not appear to have been an important factor for retardation of u this analysis suggests a ranking for u retention i microfracture network retention mesofracture retention and ii individual microfracture retention matrix retention

unsaturated seepage and subterranean holes conspectus and exclusion problem for circular cylindrical cavities this paper introduces the general theory of water exclusion from or entry into subterranean holes from steady uniform downward unsaturated seepage buried holes serve as obstacles to the flow and so increase water pressure at parts of the hole surface when downward seepage is fast enough and or the hole is large enough water pressure increases to the point where a seepage surface forms and water enters the hole contrary to the conventional picture drawn from capillary statics hydrodynamics shows that the larger the hole the more vulnerable it is to water entry cavity shape is important also applications include optimal design of configurations of tunnels and underground repositories e g for nuclear wastes against entry of seepage water the theory also embraces the disturbance of seepage flows by buried impermeable obstacles such as stones and structures the quasi linear exclusion problem for circular cylindrical cavities is solved both exact solutions and simple asymptotic results are found and graphs and tables presented the moisture field about the cavity exhibits upstream and downstream stagnation points and retarded regions roof drip lobes in which the moisture content and downward flow velocity are augmented by water essentially deflected from the cavity roof and the dry shadow region of reduced moisture content and flow velocity essentially sheltered by the cavity a practical consequence is that we can establish for any given combination of downward seepage velocity cavity radius saturated hydraulic conductivity k1 and sorptive number whether or not seepage water will enter a cylindrical cavity

physical and stable isotope evidence for formation of secondary calcite and silica in the unsaturated zone yucca mountain nevada abstract calcite and silica form coatings on fracture footwalls and cavity floors in the welded tuffs at yucca mountain the potential site of a high level radioactive waste repository these secondary mineral deposits are heterogeneously distributed in the unsaturated zone uz with fewer than 10 of possible depositional sites mineralized the paragenetic sequence compiled from deposits throughout the uz consists of an early stage assemblage of calcite fluorite zeolites that is frequently capped by chalcedony quartz intermediate and late stage deposits consist largely of calcite commonly with opal on buried growth layers or outermost crystal faces of the calcite coatings on steep dipping fractures usually are thin 3 mm with low relief outer surfaces whereas shallow dipping fractures and lithophysal cavities typically contain thicker more coarsely crystalline deposits characterized by unusual thin tabular calcite blades up to several cms in length these blades may be capped with knobby or corniced overgrowths of late stage calcite intergrown with opal the observed textures in the fracture and cavity deposits are consistent with deposition from films of water fingering down fracture footwalls or drawn up faces of growing crystals by surface tension and evaporated at the crystal tips fluid inclusion studies have shown that most early stage and some intermediate stage calcite formed at temperatures of 35 to 85 c calcite deposition during the past several million years appears to have been at temperatures 18 o values although others have interpreted the elevated temperatures as evidence of hydrothermal activity and flooding of the tuffs of the potential repository the authors conclude that the temperatures and fluid inclusion assemblages are consistent with deposition in a uz environment that experienced prolonged heat input from gradual cooling of nearby plutons the physical restriction of the deposits and therefore fluid flow to fracture footwalls and cavity floors and the heterogeneous and limited distribution of the deposits provides compelling evidence that they do not reflect flooding of the thick uz at yucca mountain the textures and isotopic and chemical compositions of these mineral deposits are consistent with deposition in a uz setting from meteoric waters percolating downward along fracture flow paths

climate impact of late quaternary equatorial pacific sea surface temperature variations magnesium calcium data from planktonic foraminifera in equatorial pacific sediment cores demonstrate that tropical pacific sea surface temperatures ssts were 2 8 0 7 c colder than the present at the last glacial maximum glacial interglacial temperature differences as great as 5 c are observed over the last 450 thousand years changes in sst coincide with changes in antarctic air temperature and precede changes in continental ice volume by about 3 thousand years suggesting that tropical cooling played a major role in driving ice age climate comparison of sst estimates from eastern and western sites indicates that the equatorial pacific zonal sst gradient was similar or somewhat larger during glacial episodes extraction of a salinity proxy from the magnesium calcium and oxygen isotope data indicates that transport of water vapor into the western pacific was enhanced during glacial episodes

phased evolution of the south north hydrographic gradient in the south china sea since the middle miocene abstract the hydrography of the south china sea scs is characterized by a south north s n thermocline gradient in the upper water column from the northern margin of the western pacific warm pool wpwp to the sea area largely controlled by the east asian monsoon here we examine the records of planktonic foraminifers from ocean drilling program odp sites 1143 and 1146 in the southern and northern parts of the scs respectively that identify three stages of evolution of the s n hydrographic gradient in the scs since the middle miocene first the s n thermocline gradient possibly occurred in the scs for the first time during the period 11 5 10 6 ma reflected by an opposite change in the relative abundance of deep dwelling planktonic foraminiferal species in the south and north next the s n thermocline gradient weakened or even disappeared during the period 10 6 4 0 ma indicated by similar changes in the relative abundance of deep dwelling species in the south and north last the s n thermocline gradient substantially increased from about 4 0 3 2 ma marked by a major increase in abundance of deep dwelling species in the northern scs and a decrease in the southern scs based on the results of oceanic and coupled ocean atmosphere model experiments and previous studies on planktonic foraminifers in the pacific ocean it is inferred that the initial wpwp was probably formed during the period of 11 5 10 6 ma in response to the closure of the indonesian seaway the wpwp then weakened or became extremely unstable before developing its present expression about 4 0 3 2 ma induced by the emergence of the isthmus of panama our observations and model experiments support the argument that the stages in evolution of the wpwp are linked to tectonic changes in ocean gateways

paleoclimatic response of the closing of the isthmus of panama in a coupled ocean atmosphere model the paleoclimatic effects of the closure of the isthmus of panama 3 ma are investigated using a coupled atmosphere ocean model consistent with earlier ocean only modelling studies it is shown that prior to closure there is an absence of deep water formation in the north atlantic hence there is a reduction in oceanic heat transport this is largely compensated for by the atmosphere such that only small changes in total planetary heat transport occur the model climate of the north atlantic is significantly warmer after isthmus closure in addition the regions surrounding the pacific ocean and south atlantic are generally cooler while the indian ocean is generally warmer in the model present day climate finally possible relationships to glaciation and initiation of northern hemisphere glacial cycles are discussed

changes in terrestrial ecosystem since 30 ma in east asia stable isotope evidence from black carbon in the south china sea a 30 m y stable isotopic record of marine deposited black carbon from regional ter restrial biomass burning from the northern south china sea reveals photosynthetic path way evolution for terrestrial ecosystems in the late cenozoic this record indicates that c3 plants negatively adjusted their isotopic discrimination and c 4 plants appeared grad ually as a component of land vegetation in east asia since the early miocene a long time before sudden c4 expansion occurred during the late miocene to the pliocene the changes in terrestrial ecosystems with time can be reasonably related to the evolution of east asian monsoons which are thought to have been induced by several intricate mechanisms during the late cenozoic and could contribute significantly to the post miocene marine carbonate isotope decline

isotopic paleoceanography of the caribbean and east pacific role of panama uplift in late neogene time comparisons of carbon isotopic data on benthic foraminifera from deep sea drilling project sites 502 western caribbean and 503 eastern pacific indicate that the difference between the atlantic and the pacific in the per mil enrichment in carbon 13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago when the difference reached the modern level 1 per mil comparisons of planktonic foraminiferal oxygen isotopic data for the caribbean and the pacific suggest that the salinity of caribbean surface waters began increasing 4 million years ago possibly in response to shoaling of the panama isthmus these results suggest that modern circulation patterns in the caribbean and eastern pacific developed by 3 million years ago in concert with changing tectonic climatic and biogeographic patterns

a balanced model of the southern central andes to explain the actual crustal thickness of the southern central andes a balanced cross section at 21 s latitude was modeled a forward modeling procedure was used on a crustal scale to explain the gross crustal structure obtained by seismic refraction investigations in the model 320 km of shortening since late cretaceous based on balanced geological cross sections are assumed for the whole andes from the trench to the foreland and the initial crustal thickness varies between 30 and 40 km it results that about 20 of the actual crustal volume must be explained by other sources than crustal shortening structures of high velocity material in the upper crust of the eastern cordillera can be explained by detached and overthrust material from the crustal base thus producing a crustal doubling in the back arc underplating of continental material that was removed by subduction erosion and magmatic addition of derivates of the subducted nazca plate may contribute to the lower crust of the forearc and arc regions crustal thickening of the eastern cordillera and parts of the altiplano was undoubtedly produced by tectonic thickening whereas the sources of thickening in the western cordillera and forearc region are uncertain

natural fission reactors in the franceville basin gabon a review of the conditions and results of a critical event in a geologic system abstract natural nuclear fission reactors are only known in two uranium deposits in the world the oklo and bangombedeposits of the franceville basin gabon since 1982 five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative european program new geological mineralogical and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2 0 ga years the franceville basin and the uranium deposits remained geologically stable over a long period of time therefore the sites of oklo and bangombeare well preserved for the reactors two main periods of actinide and radionuclides migration have been observed during the criticality under p t conditions of 300 bars and 400 500 c respectively and during a distention event which affected the franceville basin 800 to 900 ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors new isotopic analyses on uranium dioxides clays and phosphates allow us to determine their respective importance for the retention of fission products the uo 2 matrix appears to be efficient at retaining most actinides and fission products such as rees y and zr but not the volatile fission products cd cs xe and kr nor rb sr and ba some fissiogenic elements such as mo tc ru rh pd and te could have formed metallic and oxide inclusion in the uo 2 matrix which are similar to those observed in artificial spent fuel clays and phosphate minerals also appear to have played a role in the retention of fissiogenic rees and also of pu

vertical thermal structure evolution of miocene surface waters western equatorial pacific dsdp site 289 abstract time series analyses from 18 7 to 5 8 ma of the oxygen isotopic composition of four shallow dwelling planktic foraminifera globorotalia mayeri dentoglobigerina altispira globoquadrina baroemoenensis andglobigerinoides immaturus one deep dwelling planktic foraminifer globoquadrina venezuelana and a benthic foraminifer cibicidoides spp from deep sea drilling project site 289 2206 m present day depth are used to determine the evolution of vertical thermal structure in the western equatorial pacific comparisons of composite oxygen isotopic curves representing the surface mixed layer the thermocline and deep waters reveal three distinct intervals of stability in the position and strength of the thermocline during the miocene 18 7 to 16 1 ma 11 1 to 9 9 ma and 7 5 to 5 8 ma separated by two intervals of instability of the thermocline 16 1 to 11 1 ma and 9 9 to 7 5 ma from 18 7 to 16 1 ma vertical thermal gradients were the weakest of the miocene interval studied and the water column was highly stable vertical thermal gradients strengthened between 16 1 and 11 1 ma as equatorial surface waters warmed and deep waters cooled during an inferred major growth phase of the east antarctic ice sheet as a consequence the strength and position of the thermocline became highly variable this was followed from 11 1 to 9 9 ma by relative climatic stability well defined vertical thermal gradients and a slight warming of the water column by 9 9 ma increasing restriction of surface water flow through the indonesian seaway led to the piling up of warm surface waters in the western equatorial pacific and a deepening of the thermocline deep waters continued to cool and vertical thermal gradients reached a miocene maximum by 7 5 ma from 7 5 to 5 8 ma vertical thermal gradients were stable and the water column became highly stratified and marked by a thick warm surface layer and a deep well defined thermocline based upon a previous assumption that ice volume growth during the middle miocene 16 1 to 11 1 ma contributed to an increase in 18o values of 0 5 to 0 7 we have estimated water column temperature changes at site 289 between 18 7 and 5 8 ma surface waters are inferred to have warmed 2 to 4 c between 18 7 and 5 8 ma with about half of the warming occurring from 16 1 to 11 1 ma between 11 1 and 9 9 ma the entire water column warmed and from 9 9 to 7 5 ma the mixed layer expanded and the thermocline deepened as warm waters piled up in the western equatorial pacific in response to the closure of the indonesian seaway the net decrease of thermoclinal 18o values between 11 1 and 7 5 ma reflects an upward change in position of the deep dwelling planktic foraminifera globoquadrina venezuelana relative to the position of the thermocline as the thermocline deepened after 7 5 ma relative 18o values indicate thatgloboquadrina venezuelana was a deep dwelling mixed layer species deep waters 2400 to 2200 m at site 289 cooled 2 to 4 c between 18 7 and 5 8 ma with about half of the cooling occurring from 16 1 to 11 1 ma and the other half from 9 9 to 7 5 ma

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

experimental mixing variability in intersecting natural fractures laboratory experiments of flow through epoxy replicas of intersecting natural fractures show that solute transport deviates significantly from predictions of two dimensional streamline routing through parallel plate intersections surface roughness in intersecting fractures causes two major discrepancies between mixing experiments and the parallel plate predictions first the fluids exiting the intersection are not uniformly mixed but consist of ribbons with varied solute concentrations which tend to follow streamlines in the flow some of these streams maintain a nearly pure inlet fluid composition as they exit the intersection and traverse the outlet fracture second when the outlet fluids are collected and homogenized we find that this complex redirection of streamlines within natural rough walled fracture intersections results in more total mixing than is predicted by the parallel plate model

carbon reservoir changes preceded major ice sheet expansion at the mid brunhes event the beginning of the mid brunhes event ca 430 ka coincided with the largest amplitude change in d 18 o in the global ocean over the past 6 m y this large d 18 o change recorded a major ice sheet expansion that cannot be explained by small changes in orbital forcing our recent studies at ocean drilling program site 1143 from the south china sea show that this large d 18 o change was preceded by a significant negative d 13 c shift a global survey of long deep sea records has revealed periodic d 13 cmax episodes i e maximum positive values of d 13 c and both major ice sheet expansion events in the pleistocene the mid brunhes event and the middle pleistocene revolution were preceded by d 13 cmax ep isodes followed by negative d 13 c shifts this new finding suggests that disturbance in carbon reservoirs leads to major growth of ice sheet size and challenges the prevalent concept of arctic control of glacial cycles because earth is now passing again through a d 13 cmax episode it is crucial to understand the causal relationship between the successive d 13 c changes and ice sheet growth events