three dimensional variable viscosity convection of an infinite prandtl number boussinesq fluid in a spherical shell we investigate a three dimensional spherical shell model of mantle convection with strongly temperature dependent viscosity numerical calculations of convection in an infinite prandtl number boussinesq fluid heated from below at a rayleigh number of ra 10 5 are carried out for the isoviscous case and for a viscosity contrast across the shell of 1 000 in the isoviscous case convection is time dependent with quasi cylindrical upflow plumes and sheet like downflows when viscosity varies strongly across the shell convection is also time dependent but major quasi cylindrical downflows with spider like extensions occur at both poles and interconnected upflow plumes occur all around the equator the surface expression of mantle convection in the earth downwelling sheets at trenches upwelling plumes at hot spots and upwelling sheets at midocean ridges resembles structures seen in both the isoviscous and variable viscosity models the dominance of spherical harmonic degree l 2 in the variable viscosity model agrees with the l 2 dominance in the earth s geoid topography and seismic tomography the overall pattern of convection in the variable viscosity case is similar to the distribution of major highlands and volcanic rises on venus

recent climatic significance of chemical signals in a shallow firn core from an alpine glacier in the south asia monsoon region abstract there are 19 sub tropical temperate glaciers on mount yulong the southernmost currently glacier covered area in eurasia controlled by the south western monsoon climate in the summer of 1999 a firn core 10 10 m long extending down to glacier ice was recovered in the accumulation area of the largest glacier baishui no 1 periodic variations of climatic signals above 7 8 m depth were apparent and net accumulation of four years was identified by the annual oscillations of isotopic and ionic composition the boundaries of annual accumulation were confirmed by higher values of electrical conductivity and ph and by dirty refreezing ice layers at the levels of summer surfaces calculated mean annual net accumulation from 1994 95 to 1997 98 was about 900 mm water equivalent the amplitude of isotopic variations in the profile decreased with increasing depth and isotopic homogenization occurred below 7 8 m as a result of meltwater percolation variations of 18 o above 7 8 m showed an approximate correlation with the winter climatic trend at lijiang station 25 km away concentrations of ca 2 and mg 2 were much higher than those of na and k indicating that the air masses for precipitation were mainly from a continental source and that the core material accumulated during the winter period the close correspondence of cl and na indicated their common origin the decreasing trend of na cl ratios with increasing depth further reflects a progressive homogenization process caused by meltwater percolation concentrations of so 4 2 and no 3 in the core are quite low the mean annual net accumulation in the core and the estimated ablation indicate that the average annual precipitation above the glacier s equilibrium line is 2400 3100 mm but this needs to be confirmed by long term observation of mass balance

transitions in thermal convection with strongly variable viscosity one of the most important material properties influencing the style of convection in the mantles of terrestrial planets is the extreme temperature dependence of viscosity three dimensional numerical convection calculations in a wide 8 8 1 cartesian box and in a spherical shell ratio of inner to outer radius of 0 55 characteristic of terrestrial planets both display two fundamental transitions as the viscosity contrast is progressively increased from unity to a factor of 105 these transitions not only mark changes in the style of deformation in the upper boundary layer from mobile lid to sluggish lid to stagnant lid but also have dramatic effects on the style planform and horizontal length scales of convection in the entire domain vertical variations of viscosity are the most important for determining the horizontal length scales of the convective patterns while lateral viscosity variations play a role in shaping the relative structures of the upwelling and downwelling flows convection in venus appears to be represented most closely by the sluggish lid regime of convection whereas the earth with plate tectonics more closely resembles the mobile lid style of convection forcing plate like characteristics onto the convective flows in the form of imposed weak zones and prescribed surface velocities results in flow patterns dominated entirely by the form used to enforce the plate like behavior and tells us little about why the mantle exhibits long wavelength heterogeneity

the evolution and environmental significance of glaciochemistry during the ablation period in the north of tibetan plateau china in order to explore the glaciochemistry of shiyi glacier in the central qilian mountains a total of 142 samples have been collected for fresh snow surface snow snowpit and meltwater from july 2012 to november 2013 the results indicated cations dominated the glaciochemistry especially the high ca2 concentration relatively high cl and na concentrations were mainly contributed by desert dust aerosols and the atmospheric aerosols from warm water bodies such as the black caspian and aral seas the analysis also confirmed the chemical ions in fresh snow are mainly contributed by atmospheric wet deposition surface snow is mainly influenced by dry deposition and snowpits are influenced by both atmospheric deposition and leaching process while meltwater is influenced by water rock interactions the profile of ionic concentration in the snowpit became more smoothed during ablation period owing to the influence of leaching the elution sequence of the snowpit in the shiyi glacier was so42 k ca2 mg2 na cl no3 the chemical concentrations for meltwater are mainly influenced by water dilution and soil thawing the ions in snow in the central qilian mountains showed relatively lower concentrations than in the western qilian mountains tianshan mountains and altai mountains it was closer to balance than in glaciers in the central tibetan plateau but was higher than in the southern tibetan plateau different ionic sources and dissolution process had caused the differences of chemical composition for fresh snow surface snow snowpit and meltwater and these chemical compositions reflected the atmospheric background values in the northern tibetan plateau c 2014 elsevier ltd and inqua all rights reserved

mantle convection models featuring plate tectonic behavior an overview of methods and progress abstract arguably the presence of plate tectonic type surface motion for periods that endure over hundreds of millions of years is the primary feature a mantle convection model must possess in order to be considered earth like from the early days of mantle dynamics modeling research has been dedicated to understanding how mantle convection produces the first order observations of plate tectonics as well as how the plates and deep mantle interact fledgling studies of the effect of plates on the mantle recognized the ability of imposed plate scale surface motion to influence global temperatures and heat flux and organize convective planform later studies featuring model plates with dynamically determined velocities discovered that the interaction between convection and the plates could result in cyclic plate motion patterns and other time dependent behavior that was not manifested in systems in which dynamic plates were absent focussing on different aspects of system realism with respect to terrestrial mantle convection has spawned multiple approaches for modeling convection with dynamic integrated plates in broadest terms the two main approaches can be categorized as rheological modeling methods and methods utilizing evolving surface boundary conditions over the past dozen years studies focussing on the former approach have steadily made progress in modeling the self generation of plate tectonics from convection dynamics continual advances have been encouraging and a consensus is beginning to form regarding the necessary requirements for obtaining the primary elements of plate like surface motion however despite significant progress the generation of plates over long periods has not yet been modeled with earth like convective vigor in contrast models utilizing dynamically determined boundary conditions to achieve plate like surface motion have relatively little difficulty with emulating terrestrial convective vigor or simulations of billions of years instead their weakness is more fundamental they can only provide insight into the reciprocating dynamics of the mantle and plates once the existence of the plates is assumed and they cannot model any aspects of the dynamics responsible for the origin of the plates this paper briefly reviews the evolution of mantle convection models featuring plates and examines the progress that has been made in our understanding of the feedback between the mantle and plate tectonics through the use of both rheological and boundary condition modeling methods common findings recent advances and unbridged problems are identified and discussed

association between atmospheric circulation patterns and firn ice core records from the inilchek glacierized area central tien shan asia 1 glacioclimatological research in the central tien shan was performed in the summers of 1998 and 1999 on the south inilchek glacier at 5100 5460 m a 14 36 m firn ice core and snow samples were collected and used for stratigraphic isotopic and chemical analyses the firn ice core and snow records were related to snow pit measurements at an event scale and to meteorological data and synoptic indices of atmospheric circulation at annual and seasonal scales linear relationships between the seasonal air temperature and seasonal isotopic composition in accumulated precipitation were established changes in the 18o air temperature relationship in major ion concentration and in the ratios between chemical species were used to identify different sources of moisture and investigate changes in atmospheric circulation patterns precipitation over the central tien shan is characterized by the lowest ionic content among the tien shan glaciers and indicates its mainly marine origin in seasons of minimum precipitation autumn and winter water vapor was derived from the arid and semiarid regions in central eurasia and contributed annual maximal solute content to snow accumulation in tien shan the lowest content of major ions was observed in spring and summer layers which represent maximum seasonal accumulation when moisture originates over the atlantic ocean and mediterranean and black seas

assessing li and other leachable geochemical proxies for paleo salinity in lake sediments from the mono basin ca usa abstract regional climate driven hydrological changes are accompanied by salinity changes in closed basin lakes we have investigated acid leachable li along with other leachable ions including mg ca and sr as geochemical proxies of salinity in lake sediments in the mono basin california all the elements in the acid leachable suite show a strong correlation with paleo lake level estimates based on physical and stratigraphic evidence the caco3 content of lake sediments which has been shown to be a reliable proxy for lake level changes in the mono basin and the adjoining owens lake basin corresponds well with our acid leachable proxy data

glaciochemistry of polar ice cores a review human activities have already modified the chemical composition of the natural atmosphere even in very remote regions of the world the study of chemical parameters stored in solid precipitation and accumulated on polar ice sheets over the last several hundred thousand years provides a unique tool for obtaining information on the composition of the preindustrial atmosphere and its natural variability over the past this paper deals with the chemistry of polar ice focused on the soluble mineral na nh4 k ca mg h f cl no3 so4 and h2o2 and organic methanesulfonate ch3so3 formate hcoo acetate ch3coo and formaldehyde hcho species and their interpretation in terms of past atmospheric composition aerosols and water soluble gaseous species we discuss ice core dating the difficulties connected with trace measurements and the significance of the ionic composition of snow we examine temporal from the last decades back to the last climatic cycle and spatial including examples from coastal as well as central areas of greenland and antarctica variations in the ionic budget of the precipitation and evaluate ice core studies in terms of the chemical composition of our past atmosphere we review 1 how greenland and antarctic ice cores that span the last few centuries have provided information on the impact of human activities and 2 how the chemistry of deep ice cores provides information on various past natural phenomena such as climatic variations glacial interglacial changes el nino volcanic eruptions and large boreal forest fires

soluble ionic and oxygen isotopic compositions of a shallow firn profile baishui glacier no 1 southeastern tibetan plateau in the summer of 2004 a firn profile 18 3m long extending down to glacier ice was recovered in the accumulation area of the largest glacier baishui no 1 on yulong mountain the southernmost glacier covered area in mainland eurasia multivariate empirical orthogonal function eof and statistical correlation analyses of major ion data from the profile demonstrate that three distinct types of ionic material contribute to the chemical characteristics of firn in this monsoonal region material of marine origin which is transported by the indian southwest monsoon crustal materials which come from local sources and anthropogenic pollutants which are produced by industrial and agricultural activities in south asia although the influence of post depositional processes on the seasonal isotopic and soluble ionic compositions is significant dust layers in the firn profile are clearly visible due to the effects of meltwater percolation the dust layers generally coincide with latesummer snow surfaces we therefore use the dust layers combined with the seasonal variations of electrical conductivity ec ca and mg to establish a depth age scale for the firn profile the reconstructed net accumulation has a significant negative correlation with the temperature at lijiang whereas the correlation between the net accumulation and the precipitation amount at lijiang is weak although the do time series of the firn profile was modified significantly by meltwater percolation the correlation between annual mean do values and the indian southwest monsoon index wsi1 is significant this result suggests that do records from monsoon influenced temperate glaciers can provide a valuable record of past variations of the indian southwest monsoon

changes in ionic and oxygen isotopic composition of the snow firn pack at baishui glacier no 1 southeastern tibetan plateau to properly interpret climatic and environmental information recorded in ice cores retrieved in the temperate glacier region the post depositional processes especially the meltwater percolation need to be well understood from late april to mid july 2006 two successive snow pits site p1 located at 4 750 m a s l in the ablation zone and site p2 at 4 900 m a s l in the accumulation area at baishui glacier no 1 mt yulong southeastern tibetan plateau were excavated changes in soluble ions k na ca2 mg2 cl no3 and so4 2 and stable oxygen isotopes 18o in the snow pits were investigated before the melting period soluble ions and 18o exhibit pronounced seasonal variations with high values in the upper snow firn layer accumulated in winter spring season and relatively low values in the lower firn layer deposited in summer season reflecting seasonal shift in the indian summer monsoon and winter westerlies over the region during the melting period most ions in the upper snow firn layer were leached with different efficiencies although no significant changes in ion concentrations in the lower firn layer were observed the impurities in the upper snow firn layer could be eluted down into the lower firn layer however the pre melting isotopic stratigraphy remained relatively unchanged during the observation period indicating that the influence of meltwater percolation on stable isotopes is less significant than that on soluble ions this understanding is crucial for the interpretation of ice core records in the monsoon temperate glacier region

modification of three ice core δ18o records from an area of high melt stable oxygen isotope ratios d 18 o of three shallow ice cores extending back to 1963 from urumqi glacier no 1 at the headwater of urumqi river tien shan northwest china were used to test the relationship between d 18 o and contemporaneous surface air temperature ta the ice cores were dated using the seasonal stable isotopic signals and seven insoluble particulate activity horizons associated with known nuclear tests although a strong positive relationship exists between d 18 oi n precipitation and ta at our study site this relationship is not preserved between the annually averaged ice core d 18 o records and the local temperature due to post depositional modification these results indicate that the processes forming the ice core chemical records in areas of high melt must be understood before the d 18 o record can be confidently interpreted as a climatic indicator

a 36cl chronology of lacustrine sedimentation in the pleistocene owens river system we have used 36 cl to date climatically sensitive saline lake sediments from hydrologically closed basins in southeastern california primarily sediments from searles and panamint basins during wet periods of the past 2 0 m y lakes that formed in the closed basins fluctuated in size in direct response to the balance between runoff and evaporation we have compiled a chronology for the km 3 core from searles lake based on ages determined by 36 cl for the evaporites together with those determined by 14 c and u th series for younger sediments and by magnetostratigraphy for older sediments this chronology along with other criteria such as correlations between searles and panamint basins the chloride budget and sedimentology is used to reconstruct the history of lake fluctuations in the paleo owens river system we infer that searles lake desiccated at most twice since 600 ka during the interval around 290 ka and from 10 ka to the present the lake history curve shows that the holocene epoch is anomalously arid major overflows from searles to panamint occurred during the intervals between 1 3 ma and 1 0 ma 750 ka and 600 ka 500 ka and 400 ka and 150 ka and 120 ka comparing the lake fluctuation chronology to the 18 o record of marine foraminifera we infer that the strongest similarity is in the periodicities of the cycles 40 to 50 kyr before the matuyama brunhes magnetic reversal 730 ka and 100 kyr thereafter we find however that at searles lake this fluctuation in the lake chronology is modulated by longer term cycles of aridity and humidity thus although the mid latitude quaternary climate record reflects the mid to high latitude ice volume fluctuations that dominate the marine 18 o record it also contains evidence for climatic forcing of a different type

paleohydrologic regimes in the southwestern great basin 0 3 2 my ago compared with other long records of gobal climate abstract nine distinct paleohydrologic regimes in the southwestern great basin over the last 3 2 my are recorded by the lacustrine deposits in km 3 a 930 m core from searles lake california these are characterized as being wet intermediate or dry like today excepting the present incomplete regime each lasted 0 12 to 0 76 my major regime changes 0 01 0 13 0 6 and 2 5 my ago appear to coincide with recognized changes in global ice sheet histories as represented by 18 o and other records from marine sediments but comparable changes 0 3 1 0 1 3 and 2 0 my ago do not appear to coincide closely with comparable perturbations in ice sheet histories however all regime boundaries during the last 1 75 my coincide closely in time with changes in sea surface temperatures in the tropical atlantic and many coincide with other deep sea and continental paleoclimatic boundaries the average duration of these paleohydrologic regimes was about 0 4 my standard deviation 0 2 my or less depending on assumptions and it is suggested that the regime boundaries reflect times of change in global sea surface temperatures possibly controlled in part by the earth s 413 000 yr orbital eccentricity cycle during the wettest and driest regimes in the searles lake area lake levels were not sufficiently affected by the 23 000 to 42 000 or 100 000 yr climate cycles related to high latitude ice sheet fluctuations to produce changes in the lacustrine sediment character during intermediate regimes however when lacustrine sedimentation in this area was more sensitive to climate the sediments in km 3 record lake fluctuations with average frequencies near those of the ice sheets this seems to indicate that the high latitude ice sheet fluctuations caused local climatic perturbations but did not dominate the hydrologic component of climate in this area other lacustrine deposits in the southwestern great basin of california and nevada have ages comparable in part to those of the wet to intermediate regimes indicated by km 3 and they may all be products of finite periods when lake expansion alluvial fan growth increased spring discharge and fluvial deposition were promoted in this area by widespread wet climates glacier expansion in the sierra nevada may also have been primarily an expression of and in phase with these wet regimes

seasonal variability of ionic concentrations in surface snow and elution processes in snow firn packs at the pgpi site on urumqi glacier no 1 eastern tien shan china to investigate the effects of both non meltwater and meltwater related post depositional processes on chemical species within the snow firn pack a research program the program for glacier processes investigation was initiated in july 2002 by the tien shan glaciological station chinese academy of sciences the seasonal variability of the ionic concentrations in surface snow samples and ion elution behavior in the snow firn pack were assessed from surface samples collected year round and 1011 samples collected from a snow pit at weekly intervals from september 2003 through september 2004 the results indicate that elevated ionic concentrations in spring and summer result from asian dust storm derived aerosol input and other aerosols entrained in precipitation potential sources of these chemical species are explored using correlation and factor analyses the elution sequence through the snow firn pack was determined to be so4 2 c a 2 n a n o3 c l k m g 2 n h4 the elution of ions at the sampling site was found to be driven primarily by air temperature and became evident when a diurnal mean temperature of 3 68c was attained at 0 38c all of the year round new ionic input was leached from the snow

convection planforms in a fluid with a temperature dependent viscosity beneath a stress free upper boundary this study presents the results of a joint numerical and laboratory study of 3 d planforms of time dependent convection in a newtonian fluid layer with a temperature dependent viscosity planforms are found for the case in which the fluid is bounded from above by a stress free boundary and below by a rigid boundary and compared to the planforms found when the fluid layer is bounded by rigid boundaries all of the laboratory and numerical experiments were performed with nearly the same values of the rayleigh number ra1 2 1 105 and viscosity ratio r 50 and viscosity law to allow direct comparisons when the fluid layer is bounded by rigid boundaries the observed planforms are the spoke patterns which are an interconnected network of descending and ascending sheets however when the upper boundary is stress free the morphology of the downwellings change from spokes to a dendritic network of descending sheets and the wavelength of the planform is more than 3 times as large as the spoke pattern planform in both cases the numerical and laboratory experiments are in good agreement

the influence of interior mantle temperature on the structure of plumes heads for venus tails for the earth 1 to form mantle plumes with large heads and narrow trailing conduits requires large 102 viscosity variations within the hot thermal boundary layer at the plume source however new and published data from laboratory experiments show that if a layer of fluid with a temperature dependent viscosity is heated from below and cooled from above convection occurs beneath a stagnant lid and the interior fluid temperature remains close to that of the hot boundary consequently the viscosity reduction across the hot thermal boundary layer is always 10 and upwellings take the form of discrete plume heads thermals subduction of the stagnant layer equivalent to recycling lithospheric plates on the earth will cool the interior of the mantle thus increasing the viscosity ratio hence the existence of earth like mantle plumes may be a consequence of plate tectonics an absence of plate tectonics on venus may lead to upwellings composed of thermals the surface expression of which are coronae as well as a smaller core heat flux than on earth

chemical and isotopic signature of old groundwater and magmatic solutes in a costa rican rain forest evidence from carbon helium and chlorine 1 c he and cl concentrations and isotopes in groundwater and surface water in a lowland costa rican rain forest are consistent with the mixing of two distinct groundwaters 1 high solute bedrock groundwater representing interbasin groundwater flow igf into the rain forest and 2 low solute local groundwater recharged in the lowlands in bedrock groundwater high 13c 4 89 low 14c 7 98 pm high r ra for he 6 88 and low 36cl cl 17 10 15 suggest that elevated tracer concentrations are derived from magmatic outgassing and or weathering of volcanic rock beneath nearby volcan barva in local groundwater the magmatic signature is absent and data suggest atmospheric sources for he and cl and a biogenic soil gas co2 source for dissolved inorganic carbon dating of 14c suggests that the age of bedrock groundwater is 2400 4000 years most likely at the lower end of the range local groundwater has 14c 100 pm indicating the presence of bomb carbon and thus ages less than 55 years overall data are consistent with a conceptual hydrologic model originally proposed on the basis of water budget and major ion data 1 large variation in solute concentrations can be explained by mixing of the two distinct groundwaters 2 bedrock groundwater is much older than local groundwater 3 elevated solute concentrations in bedrock groundwater are derived from volcanic fluids and or rock and 4 local groundwater has not interacted with volcanic rock tracers with different capabilities converge on the same hydrologic interpretation also transport of magmatic co2 into the lowland rain forest via igf seems to be significant relative to other large ecosystem level carbon fluxes

some comments on climatic reconstructions from ice cores drilled in areas of high melt poor consideration has been given in many arctic circum polar ice core studies to the effect of summer snow melt on chemistry stable isotope concentrations and time scales many of these corps are drilled close to the firn line where melt is intense some come from below the firn line where accumulation is solely in the form of super imposed ice in all cases seasonal signals are reduced or removed and in some time gaps develop during periods of excessive melting which situate the drill site in the ablation zone consequently cross correlations of assumed synchronous events among the cores are invalid so that time scales along the same cores differ between authors by factors of over 2 many so called climatic signals are imaginary rather than real by reference to published analyses of cores from the superimposed ice zone on devon ice cap koerner 1970 and meighen ice cap koerner and paterson 1974 it is shown how melt affects all the normally well established ice core proxies and leads to their misinterpretation despite these limitations the cores can give valuable low resolution records for all or part of the holocene they show that the thermal maximum in the circum polar arctic occurred in the early holocene this maximum effected negative balances on all the ice caps and removed the smaller ones cooler conditions in the second half of the holocene have caused the regrowth of these same ice caps

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

strong influence of water vapor source dynamics on stable isotopes in precipitation observed in southern meghalaya ne india to calibrate 18o time series from speleothems in the eastern indian summer monsoon ism region of india and to understand the moisture regime over the northern bay of bengal bob we analyze the 18o and d of rainwater collected in 2007 and 2008 near cherrapunji india d values range from 18 5 to 144 4 while 18o varies between 0 8 and 18 8 the local meteoric water line lmwl is found to be indistinguishable from the global meteoric water line gmwl late ism september october rainfall exhibits lowest 18o and d values with little relationship to the local precipitation amount there is a trend to lighter isotope values over the course of the ism but it does not correlate with the patterns of temperature and rainfall amount 18o and d time series have to be interpreted with caution in terms of the amount effect in this subtropical region we find that the temporal trend in 18o reflects increasing transport distance during the ism isotopic changes in the northern bob surface waters during late ism and vapor re equilibration with rain droplets using an isotope box model for surface ocean waters we quantify the potential influence of river runoff on the isotopic composition of the seasonal freshwater plume in the northern bob temporal variations in this source can contribute up to 25 of the observed changes in stable isotopes of precipitation in ne india to delineate other moisture sources we use backward trajectory computations and find a strong correlation between source region and isotopic composition palaeoclimatic stable isotope time series from northeast indian speleothems likely reflect changes in moisture source and transport pathway as well as the isotopic composition of the bob surface water all of which in turn reflect ism strength stalagmite records from the region can therefore be interpreted as integrated measures of the ism strength

quantifying the flux of hydrothermal fluids into mono lake by use of helium isotopes in mono lake concentrations of 3 he and 4 he are greater than atmospheric equilibrium values indicating a subsurface helium source this assertion is supported by vertical concentration profiles that show maximum d 3 he values 168 occurring within the pycnocline the slope of the regression between 3 he and 4 he concentrations is near 2 8 ra ra is the atmospheric helium isotope ratio indicating that the subsurface source has a component of mantle helium hydrothermal springs and gas vents from paoha island have similar ratios which suggests that discharge from this thermal system is the most important source a 3 he mass balance indicated that below the deep pycnocline at 17 m 25 cc stp of 3 he accumulates each year 90 from the influx of hydrothermal water and 10 from the in situ decay of tritium although it is not retained because of gas transfer across the air water interface 70 cc stp of 3 he is injected each year into the upper 17 m when 3 he is used as a tracer the influx of hydrothermal water below the deep pycnocline was estimated to be 0 045 6 0 025 m 3 s 21 because the hydrothermal water is fresher than mono lake this influx causes the salinity of the monimolimnion to decrease by 0 1 salinity units per year and may play a small but important role in the salinity budget of this layer which is presently denser than and isolated from the surface water the isotopes of helium 3 he and 4 he have become important tracers for continental fluids for a number of reasons first helium isotope ratios vary significantly between the different reservoirs in the geosphere the 3 he 4 he isotope ratio of the atmosphere 1 4 3 10 26 lies between that of the deep crust 5 3 10 28 and that of the upper mantle 10 25 second continental fluids retain in their helium isotopes a record of the different reservoirs they have contacted hence in bodies of water such as lakes that receive the influx of water from many sources the helium isotope signature can be used to identify the different sources third the isotopic composition of helium can change as a result of the production of

the 36c1 ages of the brines in the magadi natron basin east africa the depression in the east african rift which includes both lake magadi and lake natron forms a closed basin within which almost all the dissolved chloride originates in precipitation since there is no important source of very ancient sedimentary chloride this provides an ideal setting for the evaluation of the sup 36 cl methodology as a geochemical and hydrological tracer the main source of recent water as represented by the most dilute samples measured is characterized by a sup 36 cl cl ratio of 2 5 times 10 sup minus 14 in agreement with the calculated value expected in precipitation surface evaporation increases the chlorinity of the local freshwater inflow by about a factor of 110 without changing the isotopic ratio indicating that little chloride enters the system in the form of sediment leachate a second type of brine found in the basin occurs in a hot deep groundwater reservoir and is characterized by lower sup 36 cl cl ratios 1 2 times 10 sup minus 14 by comparing this value with the 2 5 times 10 sup minus 14 in recent recharge one obtains an approximate salt accumulation age of 760 ka which is consistent with thee time of the first appearance of the lake these older brines also have lower sup 18 omore and sup 2 h values which indicate that they were recharged during a climatically different era the sup 36 cl cl ratios in the inflowing waters and in the accumulated brine together with the known age of the lake magadi basin may be used to estimate the importance of the hypogene and epigene as opposed to the meteoric mode of sup 36 cl production such a calculation shows that the hypogene and epigene processes together contribute less than 6 of the total sup 36 cl present in the lake less

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

tibetan plateau summer monsoon northward extent revealed by measurements of water stable isotopes a program of individual precipitation events and river water sampling and of water isotopic measurements d 18o was carried out during summer 1996 along a northeast southwest transect of the tibetan plateau the spatial distribution of both 18o and deuterium excess d d 8 18o of the precipitation reveals three distinct regions simulations with a simple isotopic model and seasonal isotopic variations measured at two extreme south and north locations support our interpretation in terms of different summer moisture origins 1 south of the himalayan mountains the moisture provided by the indian monsoon has been recycled over the indian peninsula 2 between the himalayas and the tanggula mountains the oceanic moisture is directly transported from the bay of bengal along the brahmaptra river valley 3 north of the tanggula mountains the moisture is not provided by the monsoon anymore but by continental water recycling

isotopic measurements of precipitation on central asian glaciers southeastern tibet northern himalayas central tien shan the glacial regions of central asia considered in our study are influenced by tropical monsoons and western extratropical cyclones isotopic 18o and d data were obtained over 3 years in three climatic regions gongga massif of southeastern tibet windward slope of summer monsoon xixibangma massif on the northern slope of the himalayas leeward of summer monsoon and the massifs of pobeda khan tengry in the central tien shan exposed to western airstreams the survey reported here provides information from atmospheric precipitation snow pits and a 23 m ice core the significant differences of oxygen isotopic ratios from 25 1 per mil to 9 5 per mil indicate that the indian and pacific oceans as well as the atlantic ocean are sources of moisture on the northern slope of the himalayas and southeastern tibet sharply changing isotopic ratios in precipitation corresponded to changing wind direction and were associated with different sources of air masses on the northern slope of the himalayas and southeastern tibet steady isotopic ratios in atmospheric precipitation and the absence of changing 18o composition in a 23 m ice core suggest only one source of moisture in central tien shan the relatively heavier oxygen isotopic ratios in atmospheric precipitation and ice core indicate the moisture from which the precipitation was derived originated over the caspian or mediterranean seas within the eurasian continent air masses developed over the atlantic ocean advance farther than those from the pacific and indian oceans

mantle dynamics in mars and venus influence of an immobile lithosphere on three dimensional mantle convection numerical calculations of fully three dimensional convection in constant viscosity compressible spherical shells are interpreted in terms of possible convective motions in the mantles of venus and mars the shells are heated both internally and from below to account for radiogenic heating secular cooling and heat flow from the core the lower boundary of each of the shells is isothermal and shear stress free as appropriate to the interface between a mantle and a liquid outer core the upper boundary of each of the shells is rigid and isothermal as appropriate to the base of a thick immobile lithosphere calculations with shear stress free upper boundaries are also carried out to assess the role of the rigid surface condition the ratio of the inner radius of each shell to its outer radius is in accordance with possible core sizes in both venus and mars a calculation is also carried out for a mars model with a small core to simulate mantle convection during early core formation different relative proportions of internal and bottom heating are investigated ranging from nearly complete heating from within to almost all heating from below the rayleigh numbers of all the cases are approximately 100 times the critical rayleigh numbers for the onset of convection cylindrical plumes are the prominent form of upwelling in the models independent of the surface boundary condition so long as sufficient heat derives from the core thus major volcanic centers on mars such as tharsis and elysium and the coronae and some equatorial highlands on venus may be the surface expressions of cylindrical mantle plumes the form of the downwelling sheets is significantly affected by the rigid boundary in that the sheets are more irregular in their horizontal structure than when the top boundary is shear stress free in the mainly heated from within models the downwelling sheets are also shorter and less temporally durable when the top boundary is rigid than when it is stress free thus the free motion of plates on the earth facilitates extensive durable convective currents that drive the plates while the stiffening of the lithospheres on mars and venus promotes a style of convection that is not particularly effective in breaking the lithosphere into plates in the rigid top cases the upper boundary layer surrounding the plumes appears to be interspersed with downwelling currents emanating radially from the plumes axes these currents may establish a stress field at the base of hotspot swells that could lead to radial fractures such as those on tharsis models with rigid upper boundaries have higher interior temperatures than do similar models with shear stress free upper boundaries on this basis venus not only has a higher surface temperature than earth but it would have a hotter mantle as well upwelling plumes are more numerous when the outer boundary is rigid as compared with shear stress free flows dominated by a few stong plumes occur when the proportion of basal heating is large thus if the martian crustal dichotomy were caused by a convective system dominated by spherical harmonic degree l 1 then the convection may have been driven strongly from below by a heating pulse accompanying core formation or from the flow of heat from an early hot core if a convective mechanism is responsible for the crustal dichotomy then the dichotomy is likely a very ancient feature however the small core models we consider consistently produce a convective pattern with a dominant l 2 signature which does not correlate with the martian crustal dichotomy a yet smaller core may be necessary to yield the l 1 pattern the small core convective pattern does correlate with the geoid and topography signatures of tharsis which have strong l 2 components and the model produces dynamic uplift comparable to the total topography of the tharsis rise models with larger cores i e with the probable inner to outer radius ratio of mars present mantle generate 4 km of uncompensated topography similar to estimates of the uncompensated tharsis topography thus the tharsis rise could have achieved its full height early in the evolution of mars by mantle plume dynamic uplift at present the uncompensated portion of the tharsis rise topography can be accounted for by dynamic uplift obviating the need for elastic support the present compensated portion of the tharsis topography could be attributed to volcanic or magmatic crustal thickening or depletion of the underlying mantle large core models appropriate to the present mars produce too many plumes to account for just two major volcanic centers i e tharsis and elysium mantle plume activity could be focussed beneath regions like tharsis if fracturing or thinning of the lithosphere in these regions has facilitated magma and heat transport across the lithosphere a similar mechanism could be responsible for clustering of coronae on venus there are no deep seated active linear upwellings in the venus models that could be associated with linear spreading centers in aphrodite if linear spreading is actually occurring in aphrodite the phenomenon is probably a shallow passive one similar to mid ocean ridges on earth

wave dynamics in mantle plume heads and hotspot swells laboratory experiments with thermal plumes in fluid with temperature dependent viscosity suggest that wave like instabilities can form in the horizontally flowing disk shaped head of the plume the waves propagate radially outward from the axis of the plume and appear to be most intense in a finite band near the perimeter of the plume head a simple theoretical model shows that interfacial waves in a highly viscous fluid may occur if the plume head is comprised of temperature dependent viscosity fluid that cools as it flows between two boundaries the model suggests that the waves arise as an oscillatory instability and that wave formation is most robust in the colder regions of the plume head as indicated by the experiments the theory also predicts that the instability will only occur above some critical plume head flow velocity and that mantle plume conditions are generally supercritical

thermal convection as a possible mechanism for the origin of polygonal structures on pluto s surface high resolution pictures of pluto s surface obtained by the new horizons spacecraft revealed among other surface features a large nitrogen ice glacier informally named sputnik planitia the surface of this glacier is separated into a network of polygonal cells with a wavelength of 20 40 km this network is similar to the convective patterns obtained under certain conditions by laboratory experiments suggesting that it is the surface expression of thermal convection here we investigate the surface planform obtained for different convective systems in 3d cartesian geometry with different modes of heating and rheologies we find that bottom heated systems as assumed by previous studies do not produce surface planforms consistent with the observed pattern alternatively for a certain range of rayleigh roberts number rah a volumetrically heated system produces a surface planform similar to this pattern we then combine scaling laws with values of rah within its possible range to establish relationships between the critical parameters of sputnik planitia in particular our calculations indicate that the glacier thickness and the surface heat flux are in the ranges 2 10 km and 0 1 10 mw m 2 respectively however a difficulty is to identify a proper source of internal heating we propose that the long term variations of surface temperature caused by variations in pluto s orbit over millions of year produces secular cooling equivalent to internal heating we find that this source of heating is sufficient to trigger thermal convection but additional investigations are needed to determine under which conditions it can produce surface patterns similar to those of sputnik planitia

the lithium isotopic composition of waters of the mono basin california mono lake a major closed basin alkaline salt lake in eastern california derives its water from a mixture of creeks and springs with the former providing in excess of 75 of the total the li isotopic composition of lake water has not varied significantly over a 4 year meromictic period 7li 19 5 springs are isotopically distinct groundwater springs and seeps carry water enriched in isotopically heavy li whereas thermal springs supply isotopically light 7li 50 of the li to the lake 2 the li budget is sensitively balanced on small thermal spring contributions 3 of the total spring inflow and 3 the residence time of li in the lake is 28 ka other great basin closed lakes have variable li isotopic compositions 7li from 16 7 to 23 7 all of which differ significantly from those of several major lakes and seawater homogeneously 32

saline groundwater in mono basin california 2 long term control of lake salinity by groundwater mono lake is a perennial closed basin lake that has existed for at least 700 kyr this 46 m deep alkaline lake s present concentration is about 90 000 ppm total dissolved solids well logs show that beneath part of the lake saline groundwater of concentration 18 000 ppm extends to the bottom of the basin fill aquifer rogers and dreiss this issue numerical simulations indicate that redistribution of the basin s solutes between the lake and underlying saline groundwater body driven by late quaternary lake level changes may be responsible for the high present day solute content of mono lake at low lake levels the high lake concentration causes solute loss via free convection this might occur more rapidly through faults or fractures the saline fresh groundwater interface position reflects a balance between the saline groundwater density and the force of inflowing fresh groundwater discharging near the shoreline at higher lake stages the shoreline discharge zone moves toward the basin edge and the unrestrained saline groundwater mass subsides drawing solutes from the lake into the basin sediments falling lake levels again constrict the saline groundwater beneath the lake forcing saline water into the lake and increasing its solute content the sediment permeability below the lake is the major control on the solute transfer rate between the lake and the groundwater reservoir only the larger longer term lake stage changes cause saline groundwater movement depending on the assumed basin fill aquifer permeability equilibration of the saline groundwater and lake solute content with lake stage changes requires hundreds to thousands of years simulations suggest that the historical conditions where a more saline mono lake 50 000 90 000 ppm overlies less concentrated groundwater apparently 18 000 ppm could be due to the still present impact of late pleistocene lake high stands mono lake s historical salinity data have a large scatter but suggest a 5 decrease in the lake s solute content over the last 50 years we estimated the diffusive solute flux into lacustrine sediments for this period from core data the salinity data are consistent with a combination of solute diffusion into sediments and an equal or greater model predicted advective solute loss to groundwater

chlorine 36 in great basin waters revisited inspired by recently published chloride balance chronologies of closed basin lakes in the great basin researchers of the early 1960s attempted to estimate residence times of chloride in these lakes using 36cl unfortunately the analytical methods of the period were not capable of measuring the 36cl levels found in these waters about 20 years after the early research advances in accelerator mass spectrometry permitted 36cl measurement at the required sensitivity in this study we follow up on those pioneering efforts by remeasuring and reevaluating the 36cl content at several of the previously studied sites focusing on mono lake in eastern california our data show that in general the streams in the region have 36cl cl ratios similar to those expected in present day atmospheric fallout but that the terminal lakes into which the streams flow have much lower ratios these lower ratios could result from either a very long 1 million years residence time of the chloride in the basin sinks or from subsurface influx of low 36cl chloride in the case of mono lake a mass balance model based on the 36cl data and on independent estimates of chloride fluxes and reservoirs indicates major subsurface chloride input presumably from volcanic sources and an accumulation time in the range of 100 450 kyr the upper bound of this range is similar to the timing of a shift from long term humid to arid climate in the region and may indicate that hydrological closure of the basin was triggered by this event

a new 36cl hydrological model and 36cl systematics in the jordan river dead sea system the recent breakthrough in our ability to detect the radioactive isotope 36cl half life t 301 000 yr at natural levels by accelerator mass spectrometry1 allows the processes of salination of water systems to be studied in a new way by distinguishing the chloride content originating in young rainwaters and their subsequent evaporation from that generated by the leaching of ancient rocks results for the jordan river dead sea system show that the amount of chloride leached from rocks ranges from 70 in source springs to 90 in water bodies downstream furthermore the amount of water left after evaporation decreases from 50 in the source springs to 20 in the intermediate lake kinneret in the terminal dead sea 99 of the stable chloride originates from ancient rocks and evaporite formations while 80 of its 36cl content is of meteoric origin using 36cl measurements we estimate the accumulation time of the dead sea salt to be 19 000 25 000 yr

ratios of mg2 na in snowpack and an ice core at austfonna ice cap svalbard as an indicator of seasonal melting snowpack and ice core samples were collected from the dome of austfonna ice cap svalbard in the spring of both 1998 and 1999 the samples were analyzed for anions cations ph liquid electrical conductivity and oxygen isotopes concentrations of chemical components in snowpack with a history of melting were much lower than those in unmelted snowpack there was a clear difference between mg 2 na ratios previously in melted snowpack 0 011 0 02 and in unmelted snowpack 0 11 0 02 we propose that the mg 2 na ratio can be used as an indicator of whether or not firn or bubbly ice in the austfonna ice core has experienced melt percolation the mg 2 na ratio indicates that firn or bubbly ice prior to ad 1920 was much less affected by melt percolation than firn or bubbly ice formed after 1920

chemistry of snow deposited during the summer monsoon and in the winter season at baishui glacier no 1 yulong mountain china the glaciers on yulong mountain 5596 m a s l china are the southernmost in mainland eurasia the largest is baishui glacier no 1 up to 90 of the annual precipitation there falls in summer when warm moisture rich air masses associated with the southwest monsoon reach the area from the indian ocean the winter climate is influenced by air masses with a continental origin and by the southern branch of the westerlies the snow that accumulates on baishui glacier no 1 includes marine aerosols associated with the summer monsoon and dust brought in winter from central west asia africa or the thar desert area studies in may 2006 at two sites one in the accumulation area 4900 m and one in the ablation area 4750 m revealed differences between the ionic composition of the snow that had accumulated in the 2005 06 winter and that of the snow which had been deposited during the preceding summer monsoon differences in the chemistry of the summer accumulated snow at the two sites probably reflected local differences in ablation and elution rates rather than differences in ion supplies differences in the chemistry of the winter accumulated snow may reflect the influence of up valley winds which bring more crustal material to the lower site

the stable isotope evolution in shiyi glacier system during the ablation period in the north of tibetan plateau china in order to firstly explore the stable isotope evolution of shiyi glacier system in the central qilian mountains a total of 167 samples have been collected from newly deposited snow surface snow snowpits meltwater and glacier fed river water from july 2012 to november 2013 the results indicated that only the newly deposited snow in 23 november 2013 showed a significant altitude effect whereas surface snow had a reverse altitude effect owing to evaporation and sublimation in summer 2012 and 2013 the d

ikaite precipitation by mixing of shoreline springs and lake water mono lake california usa abstract metastable ikaite caco3 6h2o forms abundantly during winter months along the south shoreline of mono lake where shoreline springs mix with lake water ikaite precipitates because of its decreased solubility at low temperature and because of orthophosphate ion inhibition of calcite and aragonite during the spring some of the ikaite is transformed to anhydrous caco3 and is incorporated into tufa but most is dispersed by wave action into the lake where it reacts to form gaylussite na2ca co3 2 5h2o spring waters have low ph values are dominantly ca na hco3 have low radiocarbon activities and are mixtures of deep seated geothermal and cold groundwaters chemical modeling reveals that precipitation of caco3 can occur over a broad range of mixtures of spring and lake water with a maximum production occurring at 96 spring water and 4 lake water under these conditions all the ca and a significant fraction of the co3 of the precipitate is spring supplied a radiocarbon age of 19 580 years obtained on a natural ikaite sample supports this conclusion with the springs supplying a large and probably variable portion of the carbonate and with apparent 14c age of the carbonate varying from spring to spring tufa of similar actual antiquity may yield significantly different 14c dates making tufa at this location unsuitable for absolute age dating by the radiocarbon method

chronology of expansion and contraction of four great basin lake systems during the past 35 000 years abstract during the past 35 000 years lake bonneville lake russell and lake searles underwent a major period of lake level change the lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15 000 and 13 500 yr b p the rise of lake lahontan was gradual but not continuous in part because of topographic constraints intrabasin spill lake lahontan also had an oscillation in lake level at 15 500 yr b p radiocarbon age estimations for materials that were deposited in the lake basins indicate that lake bonneville rose more or less gradually from 32 000 yr b p and had major oscillations in level between 23 000 and 21 000 yr b p and between 15 250 and 14 500 yr b p lake russell and lake searles had several major oscillations in lake level between 35 000 and 14 000 yr b p the timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states all four lakes may have had nearly synchronous recessions between about 14 000 and 13 500 yr b p after the recessions the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11 500 and 10 000 yr b p these data provide circumstantial evidence that the younger dryas event affected climate on at least a hemispheric scale during the holocene the four lakes remained at low levels and small oscillations in lake level occurred an important aspect of the lake level data is the accompanying expansion of lake surface area at the time of the last highstand lake bonneville and lake lahontan had surface areas about 10 times larger than their mean historical reconstructed areas whereas lake russell and lake searles had surface areas about 5 times larger than their mean historical reconstructed areas differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream or they may have resulted from lake atmosphere feedback processes

spatial and seasonal variation of major ions in himalayan snow and ice a source consideration abstract the spatial and temporal variation of major ions ca2 mg2 na k nh 4 so 4 2 no 3 and cl in himalayan snow and ice is investigated by using two snow pits from the east rongbuk glacier 28 01 n 86 58 e 6500 m a s l one snow pit from the nangpai gosum glacier 28 03 n 86 39 e 5700 m a s l one snow pit from the gyabrag glacier 28 11 n 86 38 e 6303 m a s l and three ice cores from the sentik 35 59 n 75 58 e 4908 m a s l dasuopu 28 33 n 85 44 e 7000 m a s l and east rongbuk 27 59 n 86 55 e 6450 m a s l glaciers respectively in general the major ions show a significant seasonal variation with high concentrations during the non monsoon pre monsoon and post monsoon season and relatively low concentrations during the monsoon season monsoon precipitation with high local regional dust loading related to summer circulation is possibly responsible for the high concentrations occurring sporadically during the monsoon season the crest of the himalayas is an effective barrier to the spatial distribution of na cl and nh 4 concentrations but not to the major ions associated with dust influx e g ca2 and mg2 atmospheric backward trajectories from the hysplit 4 model used in identifying chemical species sourcing suggest that the major ions in the himalayan snow and ice come mainly from the thar desert located in the north india as well as west asia or even the distant sahara desert in the north africa during the winter and spring seasons this is different from the conventionally assumed arid and semi arid regions of the central asia factors such as different vapor sources due to atmospheric circulation patterns and geographical features e g altitude topography may contribute to the differences in major ionic concentrations between the western and eastern himalayas

oxygen isotopes in glacier river water austre okstindbreen okstindan norway there is no discernible river discharge from austre okstindbreen norway in winter any water formed by basal melting is likely to be stored in isolated cavities or sediments at the bed in summer a baseflow component of discharge relatively depleted of 18 o is diluted by water more enriched in 18 o glacier ice meltwater with a high 18 o value passes rapidly through channelized systems and emerges in the glacier river within a few hours headward extension of the systems in late summer may tap water stored within the glacier since the previous summer s close down stored water also is released during early melt season events when the low capacity drainage systems cannot accommodate inputs the high 18 o value indicates that the probable source of the released water is glacier ice or low altitude snowfall stratigraphic variations of 18 o values resulting from changing weather conditions characterize the accumulating winter snowpack during the melt season the mean 18 o value of the residual pack rises the water leaving it depleted of 18 o passes slowly through the glacier smoothing out variations of supply 18 o values of rain vary between and within events and their effect on glacier river 18 o values is unpredictable

saline groundwater in mono basin california 1 distribution mono lake california is a perennial closed basin saline lake up to 2 km of sediments have accumulated below the lake and a well log shows that saline groundwater of concentration 18 000 ppm extends to the bottom of the basin fill we investigated the groundwater system with the variable density flow and solute transport code sutra to determine if the basin s recharge and inferred groundwater salinity are consistent with observational data steady state model predictions of the position and shape of the interface separating saline from fresh groundwater are consistent with the salinity profile derived from the spontaneous potential sp log of a geothermal well on the lake s shoreline we also inferred the basin wide saline groundwater distribution and concentration from experiments with a steady state model of the entire basin groundwater system hydrologic variations around the lake determine the position of the saline fresh groundwater interface higher recharge rates characteristic of the sierra nevada shoreline push the interface far beneath the lake the interface probably lies below the shoreline around much of the rest of the lake on the low recharge northeastern shoreline the interface top may lie outside the lake edge this positions the saline groundwater discharge zone just below the playa surface and contributes to development of salt flats on the recently exposed former lake bed simulations suggest that the basin fill permeability may not have significant anisotropy possibly owing to extensive faulting which increases vertical permeability for an anisotropic basin fill the resultant increased channeling of recharge beneath the lake readily overcomes the opposing force of the saline groundwater density and it is flushed out of the basin sediments a simple permeability representation of the basin lithology with low permeability below the lake and an anisotropic transition to higher permeability outside the lake reasonably represents the well and spring water salinity observations

three dimensional convection planforms with internal heat generation thermal convection planforms in an infinite prandtl number isoviscous fluid layer with basal and internal heating are determined using three dimensional finite difference calculations with basal heating only at rayleigh number ra sub t 1 5 times 10 sup 5 the planform consists of a weakly time dependent spoke pattern with connected networks of equally buoyant up and downwellings with volumetric heat sources the planform is strongly time variable it consists of negatively buoyant flow concentrated in thermal trenches discrete arcuate sheets and spots cylindrical columns separated by diffuse nonbuoyant upwellings rapid planform changes occur primarily by trench propagation as observed experimentally by weinstein and olson this issue the analogy between thermal trenches in internally heated convection and convergent plate boundaries on earth suggests that the global pattern of subduction may evolve by propagation of wadati benioff zones along the strike of the slab

holocene late pleistocene climatic ice core records from qinghai tibetan plateau three ice cores to bedrock from the dunde ice cap on the north central qinghai tibetan plateau of china provide a detailed record of holocene and wisconsin wurm late glacial stage lgs climate changes in the subtropics the records reveal that lgs conditions were apparently colder wetter and dustier than holocene conditions the lgs part of the cores is characterized by more negative 18 o ratios increased dust content decreased soluble aerosol concentrations and reduced ice crystal sizes than the holocene part these changes occurred rapidly 10 000 years ago in addition the last 60 years were apparently one of the warmest periods in the entire record equalling levels of the holocene maximum between 6000 and 8000 years ago

on the effect of continents on mantle convection at the earth s surface continents and oceans impose different thermal boundary conditions at the top of the mantle laboratory experiments are used to investigate the consequences of this for mantle convection the upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle beneath this lid the thermal boundary condition was of the mixed type with a biot number depending on the dimensions and thermal conductivity of the lid experimental values of the biot number were scaled to earth values experiments were run for a large range of rayleigh numbers from 104 to 107 and for several lid widths the effects of temperature dependent viscosity and of the shape of the lid were investigated at steady state in all cases there is an upwelling beneath the conductive lid which feeds two symmetrical and elongated convective cells three different dynamic regimes were identified as a function of rayleigh number independently of the lid width at rayleigh numbers lower than 1 2 105 the upwelling is steady both in geometry and temperature structure at rayleigh numbers between 1 2 105 and 2 106 this central upwelling is intermittent at larger values of the rayleigh number there is no longer a simple upwelling structure but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent thus the conductive lid always imposes a large scale flow pattern the length of these convective cells is a function of lid width it is equal to the lid width at large values and decreases to the rayleigh benard value as the lid width decreases to zero a fluid loop model explains the most important features of this form of convection the cell length is such that the upwelling temperature is minimized for a given rayleigh number and lid width and is an increasing function of lid width and a decreasing function of rayleigh number transient experiments demonstrate that the large scale flow structure develops rapidly with even small horizontal temperature differences implications for the earth are that large scale convection cells exist in conditions which in the absence of continents would probably lead to a chaotic convection pattern dominated by plumes at high rayleigh number continental breakup is effected by a large scale line upwelling structure which includes a number of individual plumes

hydrologic issues in arid unsaturated systems and implications for contaminant transport analysis of unsaturated flow and transport in arid regions is important not only in water resource evaluation but in contaminant transport as well partic ularly in siting waste disposal facilities and in remediat ing contaminated sites the water fluxes under consid eration have a magnitude close to the errors inherent in measuring or in calculating these water fluxes which makes it difficult to resolve basic issues such as direction and rate of water movement and controls on unsaturated flow the purpose of this paper is to review these issues on the basis of unsaturated zone studies in arid settings because individual techniques for estimating water fluxes in the unsaturated zone have limitations a variety of physical measurements and environmental tracers should be used to provide multiple independent lines of evidence to quantify flow and transport in arid regions the direction and rate of water flow are affected not only by hydraulic head gradients but also by temperature and air pressure gradients the similarity of water fluxes in a variety of settings in the southwestern united states indicates that vegetative cover may be one of the pri mary controls on the magnitude of water flow in the unsaturated zone however our understanding of the role of plants is limited most unsaturated flow in arid systems is focused beneath topographic depressions and diffuse flow is limited thick unsaturated sections and low water fluxes typical of many arid regions result in preservation of paleoclimatic variations in water flux and suggest that deep vadose zones may be out of equilib rium with current climate whereas water movement along preferred pathways is common in humid sites field studies that demonstrate preferential flow are restricted mostly to fractured rocks and root zones in arid regions results of field studies of preferential flow in humid sites generally restricted to the upper 1 2 m because of shallow water tables cannot be applied readily to thick vadose zones in arid regions

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

the dependence of convection planform on mode of heating a fundamental property of a convecting fluid is its planform the distribution in the horizontal plane of hot rising regions and cold sinking regions for the earth s mantle the planform might he visualized as a map of subduction zones hotspots and possibly ocean ridges here i report numerical experiments of convection at high rayleigh number which show a strong dependence of planform on heating mode when heat generation is distributed uniformly through the box the preferred planform consists of an ensemble of time dependent cold axial sinkers distributed in a hot diffuse upward flow when half of the heat is generated within the box and the other half is input through the base the preferred planform consists of an array of hot axial plumes and elongated cold sheets in the former case the mean horizontal wavelength is about equal to the layer depth for the latter it is about twice the layer depth

the radiocarbon budget for mono lake an unsolved mystery abstract since 1957 the 14 c c ratio of the dissolved inorganic carbon in mono lake has risen by about 60 the magnitude of this increase is about four times larger than that expected from the invasion of bomb produced 14 c from the atmosphere we have eliminated the following explanations 1 measurement error 2 an unusually high physical exchange rate for non reactive gases 3 inorganic enhancement of the co 2 exchange rate and 4 biological enhancement of the co 2 exchange rate clandestine disposal of waste radiocarbon remains a dark horse explanation in the course of our investigations we have uncovered evidence for at least one episodic input of radiocarbon free carbon to the lake over the last 1000 years we speculate that this injection was related to a hydrothermal event resulting from sublacustrine volcanic activity

chlorine 36 in the waters of the mcmurdo dry valley lakes southern victoria land antarctica revisited abstract we have analyzed water samples from lakes bonney fryxell and hoare in the mcmurdo dry valleys mdv area of southern victoria land antarctica these data coupled with those of earlier work provide a new view of the sources of chloride ion to these lake systems although these lakes lie within the same geologic domain and climatic regime their chemistries are very different in that the hypolimnions have varying salinities and major ion ratios the bottom waters of both lobes of lake bonney have low 36 cl cl ratios 15 derived from a probable mixture of seawater 0 10 15 and a nearby saline seep 9 10 10 15 on the other hand the 36 cl cl ratio of both the bottom water and the surface water of lake hoare the least saline of the lakes is similar to ratios found in chloride salts in the local soil and meteoric sources by the earlier work 210 vs 440 10 15 of carlson et al 1990 the water from lake fryxell has an intermediate ratio perhaps indicating a mixture of both meteoric input and older chloride our recent surface water measurements from the three lakes demonstrate a very significant decrease in the 36 cl cl ratio from the meltwater that entered the lakes in 1984 1985 these data provide evidence of the dynamic geochemical hydrological behavior of these lakes and coupled with the earlier data help to better define the importance of differing sources of solutes to the lakes

at least three scales of convection in a mantle with strongly temperature dependent viscosity abstract we studied the convective patterns developing at high rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature dependent viscosity within this sluggish lid regime three different scales of convection develop the largest convective scale is cellular with cold downwelling sheets of viscous fluid encasing hotter less viscous parts of the tank within each of those cells develop several typically 3 7 hot 3d upwelling plumes upon impinging under the cold thermal boundary layer each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank applying those results to the earth s mantle we suggest that the large scale features of mantle convection and the co existence of several scales of convection i e slabs and plumes are produced by thermal convection in a mantle with high rayleigh number and a strongly temperature dependent viscosity material this generates a convective pattern in two large scale cells the pacific and the indo atlantic boxes our experiments further suggest that what has been named the two hot superplumes i e the two seismically slow regions encased within the subduction rings are in fact each constituted of several hot instabilities moreover plumes impacts under the lithosphere should be surrounded by cold rings of small extent the asthenosphere appears therefore as the graveyard of both small scale convection and hot plumes generated in the system