geodynamics magmatism and degassing of the earth abstract in this study terrestrial degassing processes are identified formalized both on local and global scales and combined in a single model of evolution of the upper mantle which constitutes 1 3 of the silicate earth the crust and the atmosphere unknown parameters of these processes are estimated by means of numerical modelling the model envisages early intensive convection and fractionation in the upper mantle resulting in sharp degassing of its matter later relatively slow convection in this reservoir accompanied by partial melting of silicate matter in rift and subduction zones uplift of melts into the oceanic and continental crust and their degassing early formation and subsequent recycling of the continental crust and its degassing due to intracrustal magmatism and metamorphism and accumulation of gases excepting he in the atmosphere magmatic processes are considered the most important for terrestrial degassing two stages of these processes i e partitioning of the gases between melted and solid portions and partial degassing of melt yield a pronounced u shaped fractionation of noble gases helium and neon due to high solubility and accordingly conservation in partially degassed melts as well as xenon due to a high partition coefficient and accordingly conservation in residual portions are released from partially melted matter much more slowly than argon to reconcile calculated and observed noble gas data a high rate of recycling of mantle matter must be provided in the past 74 10 17 g y during the first 0 1 ae about 90 of 36 ar had been released into the atmosphere at that time later this rate decreased sharply down to its contemporary value of about 0 6 10 17 g y less than 0 3 of 36 ar is still conserved in the upper mantle the degassing process resulted in a pronounced u shaped fractionation of noble gases in this reservoir 20 ne 36 ar m 20 ne 36 ar air 3 and 130 xe 36 ar m 130 xe 36 ar air 7 4 he 3 he and 40 ar 36 ar in the upper mantle both depend on the radioactive decay of parent elements and the rate of mantle degassing note that the noble gas radiogenic degassing clock was the only one functional during early stages of the evolution of the earth the complementary reservoir continental crust was formed very early its mass attaining the contemporary value at about 4 ga self consistent concentrations of radioactive elements in the silicate earth upper mantle crust are considerably higher than recent estimates a concentration of k equal to 0 048 is determined by 40 ar amounts in the mantle the crust and mainly in the atmosphere correspondingly u becomes 0 038 ppm and th 0 152 ppm contemporary ratios of k u 13 000 and solth u 4 being assumed for all silicate reservoirs the total contemporary fluxes atoms cm 2 sec of 3 he 4he and 40 ar rad are equal to 4 3 2 0 10 6 and 0 66 10 6 respectively practically the whole 3 he flux and 1 4 of the 4 he flux are produced by upper mantle degassing the residence times for k and 40 ar rad in the upper mantle are 1 1 and 4 1 ga respectively resulting in an enormously high value for its k ar age 4 4 ga the continental crust residence times for both species k and ar are high and the crustal mean k ar age is 1 6 ga

mid holocene el nino southern oscillation enso attenuation revealed by individual foraminifera in eastern tropical pacific sediments holocene reconstructions of the el nino southern oscillation enso provide valuable perspective on its recent evolution and can be important for assessing its future optimal assessment of past enso variability requires observations from its center of action in the eastern equatorial pacific but these are limited due to paucity of high resolution paleoceanographic archives e g corals here we use a new approach to quantify past enso variance based on the oxygen isotopic composition 18 o of individual foraminifera globigerinoides ruber from deep sea sediments in the enso source region individual g ruber foraminifera behave as monthly recorders of sea surface conditions including enso extremes circumventing the lack of annual resolution in the sediments intrapopulation 18 o distributions derived with this method from a core near the galapagos islands reveal mid holocene reductions in variance of 50 requiring drastic attenuation of the enso amplitude furthermore mg ca thermometry indicates that mid holocene background conditions were accompanied by a stronger zonal temperature gradient that coincided with a northward displaced intertropical convergence zone itcz the results suggest that the position of the itcz is an important factor in the low frequency modulation of enso and could influence its future evolution

laurentide ice sheet instability during the last deglaciation the factors leading to the full retreat of ice sheets during deglaciation are debated numerical modelling suggests that the laurentide ice sheet retreated only after a threshold for warming and radiative forcing was passed in the holocene

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

rapid subtropical north atlantic salinity oscillations across dansgaard oeschger cycles geochemical and sedimentological evidence suggest that the rapid climate warming oscillations of the last ice age the dansgaard oeschger cycles were coupled to fluctuations in north atlantic meridional overturning circulation through its regulation of poleward heat flux the balance between cold meltwater from the north and warm salty subtropical gyre waters from the south influenced the strength and location of north atlantic overturning circulation during this period of highly variable climate here we investigate how rapid reorganizations of the ocean atmosphere system across these cycles are linked to salinity changes in the subtropical north atlantic gyre we combine mg ca palaeothermometry and oxygen isotope ratio measurements on planktonic foraminifera across four dansgaard oeschger cycles spanning 45 9 59 2 kyr ago to generate a seawater salinity proxy record from a subtropical gyre deep sea sediment core we show that north atlantic gyre surface salinities oscillated rapidly between saltier stadial conditions and fresher interstadials covarying with inferred shifts in the tropical atlantic hydrologic cycle and north atlantic overturning circulation these salinity oscillations suggest a reduction in precipitation into the north atlantic and or reduced export of deep salty thermohaline waters during stadials we hypothesize that increased stadial salinities preconditioned the north atlantic ocean for a rapid return to deep overturning circulation and high latitude warming by contributing to increased north atlantic surface water density on interstadial transitions

high latitude forcing of the south american summer monsoon during the last glacial the climate of the last glacial period 10 000 to 110 000 years ago was characterized by rapid millennial scale climate fluctuations termed dansgaard oeschger d o and heinrich events we present results from a speleothem derived proxy of the south american summer monsoon sasm from 16 000 to 50 000 years ago that demonstrate the occurrence of d o cycles and heinrich events this tropical southern hemisphere monsoon reconstruction illustrates an antiphase relationship to northern hemisphere monsoon intensity at the millennial scale our results also show an influence of antarctic millennial scale climate fluctuations on the sasm this high resolution precisely dated tropical precipitation record can be used to establish the timing of climate events in the high latitudes of the northern and southern hemispheres

disentangling xenon components in nakhla martian atmosphere spallation and martian interior abstract a powdered sample of nakhla was separated into 3 subsamples one was left otherwise untreated one was washed in water and one etched with hno3 removing 6 of the original mass we report results of isotopic analysis of xenon released by laser step heating on aliquots of each of these subsamples some aliquots were neutron irradiated before isotopic analysis to allow determination of i ba and u as daughter xenon isotopes and some were not there is evidence that water soluble phases contain both martian atmospheric xenon and a component with low 129xe 132xe either martian interior xenon or terrestrial atmosphere higher temperature data from unirradiated aliquots of the water and acid treated samples reveal two component mixing one is a trapped xenon component with 129xe 132xe 2 350 0 026 isotopically identical to the martian atmosphere as measured in shock glass from shergottites it is associated with leachable iodine suggesting it is trapped close to grain boundaries it may be a result of shock incorporation of adsorbed atmospheric gas the second component is best explained as an intimate mixture of martian interior xenon and spallation xenon the martian interior component is present at a concentration of 10 12 cm3 stp g 1 132xe around 40 times lower than that observed in chassigny its association with spallation xenon produced from ba and light rare earth elements suggests it is in the feldspathic mesostasis we propose that it was trapped during crystallisation and reflects the mantle source of the parental magma

direct measurement of ar diffusion profiles in a gem quality madagascar k feldspar using the ultra violet laser ablation microprobe uvlamp abstract controversy surrounding the mechanisms and controls on argon diffusion in k feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures over a range of temperatures measurements of argon diffusion profiles in a gem quality iron rich orthoclase heated in a cold seal apparatus have been undertaken in situ using an ultra violet laser ablation microprobe uvlamp technique the results agree very closely with the previously determined bulk values for benson mines orthoclase activation energy e 43 8 1 kcal mol 1 and vacuum furnace cycle heating studies of k feldspars e 46 6 kcal mol 1 however instead of defining a single activation energy e and diffusion coefficient do the data yield two sets of parameters a low temperature 550 720 c array with an e of 47 2 2 5 kcal mol 1 198 2 10 5 kj mol 1 and a do of 0 0374 0 1123 0 0281 cm2 s 1 and a high temperature 725 1019 c array with an e of 63 8 3 4 kcal mol 1 268 0 14 3 kj mol 1 and a do of 55 0 225 5 44 2 cm2 s 1 the new results closely reproduce two sets of apparent activation energies previously measured in cycle heating studies of madagascar k feldspar 40 3 and 57 3 kcal mol 1 previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion however the uv depth profile analyses indicate simple diffusion to the grain surface and importantly diffusion radii calculated by combining the uvlamp and cycle heating data are the same as the physical grain sizes used in the experiments around 1 mm vacuum furnace stepped heating experiments on slowly cooled k feldspars have been interpreted as showing diffusion radii of around 6 m and indicate complex populations of sub grains this study indicates that madagascar k feldspar and thus probably all gem quality k feldspars act as single diffusion domains and that short circuit or pipe diffusion was not an important loss mechanism an apparent diffusion compensation relationship in the stepped heating data for madagascar k feldspar implies that similar relationships seen in other k feldspars are a result of a range of diffusion mechanisms

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

a salt oscillator in the glacial atlantic 1 the concept as shown by the work of dansgaard and his colleagues climate oscillations of one or so millennia duration punctuate much of glacial section of the greenland ice cores these oscillations are characterized by 5 c air temperature changes severalfold dust content changes and 50 ppm co2 changes both the temperature and co2 change are best explained by changes in the mode of operation of the ocean in this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50 n based on this finding we suggest that the greenland climate changes are driven by oscillations in the salinity of the atlantic ocean which modulate the strength of the atlantic s conveyor circulation

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

experimental evidence for high noble gas solubilities in silicate melts under mantle pressures abstract the solubilities of ar and xe in fe free synthetic haplogranitic and tholeiitic melts were experimentally determined in the pressure range of 1 11 gpa and at temperatures between 1500 and 2000 c experiments were performed in a piston cylinder apparatus 1 3 gpa and in a multi anvil apparatus 2 11 gpa the noble gas concentrations in the quenched glasses were determined with electron microprobe as a function of pressure ar solubility increases linearly up to about 4 5 gpa where it reaches about 4 0 and 0 8 wt for the haplogranitic and tholeiitic melt respectively at higher pressure the amount of dissolved ar remains constant suggesting that some threshold concentration is reached the xe solubility in tholeiite melt exhibits a very similar pattern it increases linearly up to about 6 gpa where a threshold concentration of 0 8 wt is reached a further increase of pressure up to 11 gpa does not result in changes in xe solubility the leveling off in noble gas solubility at high pressures may imply that the interstitial sites in the melt structure suitable for the accommodation of noble gas atoms are fully occupied indeed the experimental data can be successfully reproduced with the langmuir isotherm implying a solubility model in which the gas atoms occupy a certain population of interstitial sites however the data can be equally well described by a model assuming mixing of the noble gas atoms with the oxygen atoms of the silicate melt from a thermodynamic point of view the constant noble gas solubility at high pressures simply implies that the partial molar volumes of the respective noble gas in the fluid and in the melt are equal our results differ from those of chamorro perez et al earth planet sci lett 145 1996 97 107 nature 393 1998 352 355 who reported an abrupt order of magnitude drop of ar solubility in silica and olivine melt at around 5 gpa suggesting that melt densification results in an abrupt decrease of the hole size distribution a geochemical consequence of our results is that noble gases remain incompatible elements at pressure conditions covering most of the upper mantle therefore partial melting remains an efficient process in extracting noble gases and other volatiles from the earth s mantle

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

solubility and partitioning of ne ar kr and xe in minerals and synthetic basaltic melts we have measured the solubilities of ne ar kr and xe in natural samples of anorthite diopside forsterite spinel and in synthetic basaltic melts the minerals and melts represent equilibrium pairs in the fo an di sio2 system samples were suspended in individual crucibles in a one bar flowing mixed noble gas atmosphere at 1300 or 1332 c for times ranging from 7 18 days the solubilities in the minerals increase with increasing noble gas atomic number and typical solubility values are surprisingly high samples of a particular mineral i e anorthite that came from different localities yield distinctly different and reproducible solubility values this indicates that properties intrinsic to individual samples can influence solubility noble gases are likely to be sited in lattice vacancy defects in contrast the solubilities in the melts decrease with increasing atomic number our data overlap the low end of the range defined previously for natural basalts however the dynamic range of the values from ne to xe is not as great as for the natural melts solubilities correlate well with melt molar volume but poorly with density the partition coefficients increase with increasing noble gas atomic number for all mineral melt pairs such trends imply that the terrestrial planet atmospheres were not derived from partial melting of chondritic source material significant fractionation of kr relative to xe is not observed ruling out an origin for earth s missing xe via magmatic fractionation partition coefficient absolute values are frequently near or greater than unity for example the ranges for five diopside measurements are the following dnel 0 013 0 37 darl 0 15 0 84 dkrl 0 31 2 4 and dxel 3 2 47 this indicates that magmatic transport is not an efficient mechanism for degassing the terrestrial planets our results favor a catastrophic origin for the atmospheres

erosion rates in badland areas recorded by collectors erosion pins and profilometer techniques ebro basin ne spain abstract in badland areas of the ebro basin in a semiarid climate two erosion plots 257 m2 5 slope and 128 m2 23 slope on exposed tertiary clays were monitored over two years nov 1991 nov 1993 this material is characterized by high sodium absorption ratios which lead to high soil dispersivity the dominant erosion processes in both plots are rilling and sheet erosion rainfall intensity was recorded at a weather station connected to a data logger sediment production for single events was collected in tanks and ground lowering was measured every six months by erosion pins and microtopographic profile gauge techniques significant runoff was produced only by rainfall events above 5 mm another threshold at 20 mm rain was noted for rainfalls higher than 20 mm the 23 slope plot shows a greater runoff response than the 5 one rainfall events exceeding this threshold showed a higher sediment production for the steeper slope in the relationship between precipitation and sediment concentration an envelope curve can be drawn indicating that any rainfall event of a given amount and intensity has a maximum sediment concentration which we speculate to be a function of the runoff sediment transport capacity runoff response and sediment yield in the studied plots are controlled by the rainfall and soil characteristics and their seasonal variations in both plots the erosion pins show that erosion rates in rill areas are 25 50 higher than in the interrill areas sediment yield recorded by collector devices was higher than the rates measured by erosion pins the erosion rates based on rill cross sections by profilometers were higher than the ones recorded by collectors

megafauna from the late pleistocene holocene deposits of the upper ribeira karst area southeast brazil abstract the carbonate caves of the upper ribeira valley southeastern sao paulo state brazil preserve an abundant fossil record of pleistocene holocene south american megafauna nevertheless in comparison with other localities of the country its paleontological knowledge still can be considered sparse and in need of further research this work presents an update taxonomic survey of the megafauna material collected since the beginning of the explorations in the area based on this it discusses paleobiogeographic and paleoecological questions the fossil megafauna of the upper ribeira karst region include eremotherium laurillardi nothrotherium maquinense lestodon armatus catonyx cuvieri ocnopus gracilis ahytherium aff aureum glyptodon clavipes g reticulatus hoplophorus cf toxodon spp stegomastodon waringi equidae hippidion cf tapirus terrestris panthera onca and smilodon populator only p onca and t terrestris still remain in the region despite the occurrence of some typical taxa of the pampean paleobiogeographical province the fossil assemblage recorded shows greater affinity to the mega mammal community of the intertropical region the ecology of species found suggests the previous occurrence of fairly different environmental settings than the existing one in the area there were more temperate climates both dry and wet until the current mesophytic forest was finally established the studied material does not show any kind of chrono correlation given the type of genesis of the cave accumulations these fossils depict different faunistic moments related to climate changes during the quaternary to better understand the information provided by these deposits efforts on dating and detailed taphonomic investigations should be conducted the studies of the quaternary fauna in the state of sao paulo fill an important gap in the intricate biogeographical history of the south american megafauna

sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial interglacial cycle recent studies indicate that the tropical freshwater budget of the atlantic is modulated by changes in el nine southern oscillation enso if anomalies of the atlantic freshwater balance persist on the order of decades these might have a large influence on the north atlantic thermohaline circulation thc here we present a sensitivity study in which we use a model scenario for enso behavior during the last 120 kyr to force a simplified model of the thc with corresponding freshwater exchange between the tropical atlantic and pacific if the steady state strength of the north atlantic thc is similar to the present day its response to the forcing is of the order of 1 3 sv 1 sv 10 6 m 3 s 1 no mode changes of the thc are simulated for reasonable values of the coupling constant between freshwater exchange and enso if on the other hand the steady state overturning is significantly weaker a collapse of the thc occurs due to forcing from the tropics the modeled thc variation due to tropical freshwater perturbations are compared to those resulting from middle and high latitude freshwater forcing due to long term growth and decay of northern hemisphere ice sheets it is found that both forcings are of similar amplitude except during the deglaciation when high latitude forcing dominates a possible out of phase relationship between deep water formation in the north pacific and north atlantic is also explored

cold season synoptic scale waves over subtropical south america the most active winter synoptic scale wave patterns over south america are identified using an extended empirical orthogonal function eeof technique and are physically diagnosed using composite methods results show that the leading modes of short timescale variability propagate along two main paths over the subtropical jet latitudes 308s and over the subpolar jet latitudes 608s this research focuses on the subtropical mode and its evolution over south america the observed structure of the systems associated with the subtropical mode resembles that of midlatitude baroclinic waves both cyclonic and anticyclonic perturbations display significant modifications in their threedimensional structure as they evolve over extratropical and subtropical south america while the upper level perturbations are mostly unaffected when moving eastward the lower level perturbations advance following the shape of the andes mountains and exhibit an abrupt equatorward migration at the lee side of the mountains as a result of such detachment smaller eddy heat fluxes are observed in the vicinity of the orography and consequently a weaker eddy baroclinic growth is observed once the upper level system is on the lee side the perturbations acquire a more typical baroclinic wave structure and low level intensification of the system occurs the latter is largest around 1000 km east of the orography where enhanced moisture transports from tropical latitudes along the eastern portion of the low level cyclone favor precipitation occurrence over southeastern south america those precipitation processes seem to provide a diabatic source of energy that further contributes to the strengthening of the low level cyclone in addition an intensification of the cyclone once over the ocean was found in 60 of the situations considered which is consistent with previous research suggesting an additional source of moisture and heat flux due to the warm waters of the brazil current

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

preliminary uvlamp determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts an ultra violet laser ablation microprobe uvlamp has been applied for the first time to investigate argon partition coefficients for olivines and clinopyroxenes grown from silicate melts at 1 bar argon pressure these preliminary measurements yield crystal melt partition coefficients ranging from 0 138 0 01 to 0 013 0 003 for olivine and 0 589 0 003 to 0 0016 0 0005 for clinopyroxene the higher values may indicate sub microscopic melt inclusions or some other heterogeneous distributions of non equilibrium argon in the crystals the lower values are probably more representative of true partition coefficients and fall at least an order of magnitude below the previously reported experimental data the possibility of anomalous high argon contents for crystals in previous studies is discussed in terms of surface adsorption trapped argon and early partial melting

late quaternary environmental change in the interior south american tropics new insight from leaf wax stable isotopes stable isotope analysis of leaf waxes in a sediment core from laguna la gaiba a shallow lake located at the bolivian margin of the pantanal wetlands provides new perspective on vegetation and climate change in the lowland interior tropics of south america over the past 40 000 years the carbon isotopic compositions 13c of long chain n alkanes reveal large shifts between c3 and c4 dominated vegetation communities since the last glacial period consistent with landscape reconstructions generated with pollen data from the same sediment core leaf wax 13c values during the last glacial period reflect an open landscape composed of c4 grasses and c3 herbs from 41 20 ka a peak in c4 abundance during the last glacial maximum lgm 21 ka suggests drier or more seasonal conditions relative to the earlier glacial period while the development of a c3 dominated forest community after 20 ka points to increased humidity during the last deglaciation within the holocene large changes in the abundance of c4 vegetation indicate a transition from drier or more seasonal conditions during the early mid holocene to wetter conditions in the late holocene coincident with increasing austral summer insolation strong negative correlations between leaf wax 13c and d d values over the entire record indicate that the majority of variability in leaf wax d d at this site can be explained by variability in the magnitude of biosynthetic fractionation by different vegetation types rather than changes in meteoric water d d signatures however positive d d deviations from the observed 13c d d trends are consistent with more enriched source water and drier or more seasonal conditions during the early mid holocene and lgm overall our record adds to evidence of varying influence of glacial boundary conditions and orbital forcing on south american summer monsoon precipitation in different regions of the south american tropics moreover the relationships between leaf wax stable isotopes and pollen data observed at this site underscore the complementary nature of pollen and leaf wax 13c data for reconstructing past vegetation changes and the potentially large effects of such changes on leaf wax d d signatures

noble gases in ultramafic xenoliths from san carlos arizona this work reports the results of noble gas ne ar kr xe analyses of accidental mantle xenoliths from san carlos arizona except for the addition of radiogenic 40ar and mass fractionation effects the isotopic structures of these gases are indistinguishable from atmospheric composition the absence of 129xe excesses in these rocks may reflect indirect mixing of atmospheric gases with the source region of the xenoliths the dominant influence on the noble gas abundances in the san carlos xenoliths appears to have been diffusive gas loss which may have occurred in a mantle metamorphic event or during contact with the host basanite magma evidence is presented for the partitioning of significant amounts of the heavy noble gases into fluid inclusions in the xenolith minerals the proportion of each gas in the inclusions increases with increasing atomic weight of the gas possibly reflecting solubility effects the noble gases are present in greater concentration in pyroxenes than in olivine similar to the behavior of other incompatible elements

dating fault generated pseudotachylytes comparison of 40ar 39ar stepwise heating laser ablation and rb sr microsampling analyses three different geochronological techniques stepwise heating laser ablation 40ar 39ar rb sr microsampling have been evaluated for dating fault generated pseudotachylytes sampled along the periadriatic fault system paf of the alps because pseudotachylytes are whole rock systems composed of melt clast and alteration phases chemical control from both ar isotopes cl k ca k ratios and empa analyses is crucial for their discrimination when applied to stepwise heating 40ar 39ar analyses this approach yields accurate melt related ages even for complex age spectra the spatial resolution of laser ablation 40ar 39ar analyses is capable of contrasting melt clast and alteration phases in situ provided the clasts are not too fine grained the latter of which results in integrated mixed ages without geological information elevated cl k and ca k ratios were found to be an invaluable indicator for the presence of clast admixture or inherited 40ar due to incomplete isotopic resetting during frictional melting rb sr microsampling dating did not furnish geologically meaningful ages on the basis of isotopic disequilibria among pseudotachylyte matrix phases and independent rb sr microsampling dating of cogenetic ultra mylonites the concordant 40ar 39ar pseudotachylyte ages are interpreted as formation ages the investigated pseudotachylytes altogether reveal a cretaceous to miocene history for the entire paf consistent with independent geological evidence individual faults however consistently reveal narrower intervals of enhanced activity lasting a few million years electronic supplementary material to this paper can be obtained by using the springer link server at http dx doi org 10 1008 s00410 002 0381 6

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

does the trigger for abrupt climate change reside in the ocean or in the atmosphere two hypotheses have been put forward to explain the large and abrupt climate changes that punctuated glacial time one attributes such changes to reorganizations of the ocean s thermohaline circulation and the other to changes in tropical atmosphere ocean dynamics in an attempt to distinguish between these hypotheses two lines of evidence are examined the first involves the timing of the freshwater injections to the northern atlantic that have been suggested as triggers for the global impacts associated with the younger dryas and heinrich events the second has to do with evidence for precursory events associated with the heinrich ice rafted debris layers in the northern atlantic and with the abrupt dansgaard oeschger warmings recorded in the santa barbara basin

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

moisture transport across central america as a positive feedback on abrupt climatic changes reconstruction of sea surface salinities in the eastern equatorial pacific ocean over the past 90 000 years detected strong millennial scale fluctuations that are associated with the migration of the tropical atlantic intertropical convergence zone thus millennial scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes modulating the north atlantic freshwater budget and hence north atlantic ocean circulation moisture transport from the atlantic to the pacific ocean across central america leads to relatively high salinities in the north atlantic ocean1 and contributes to the formation of north atlantic deep water2 this deep water formation varied strongly between dansgaard oeschger interstadials and heinrich events millennial scale abrupt warm and cold events respectively during the last glacial period3 increases in the moisture transport across central america have been proposed to coincide with northerly shifts of the intertropical convergence zone and with dansgaard oeschger interstadials with opposite changes for heinrich events4 here we reconstruct sea surface salinities in the eastern equatorial pacific ocean over the past 90 000 years by comparing palaeotemperature estimates from alkenones and mg ca ratios with foraminiferal oxygen isotope ratios that vary with both temperature and salinity we detect millennial scale fluctuations of sea surface salinities in the eastern equatorial pacific ocean of up to two to four practical salinity units high salinities are associated with the southward migration of the tropical atlantic intertropical convergence zone coinciding with heinrich events and with greenland stadials5 the amplitudes of these salinity variations are significantly larger on the pacific side of the panama isthmus as inferred from a comparison of our data with a palaeoclimate record from the caribbean basin6 we conclude that millennial scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes modulating the north atlantic freshwater budget and hence north atlantic deep water formation

ar diffusion and solubility measurements in plagioclases using the ultra violet laser depth profiling technique we describe the first direct measurements of ar diffusion and solubility in plagioclases using ultra violet uv laser ablation depth profiling and noble gas mass spectrometer analyses of experimentallytreated 599 1000 8c 50 200 mpaofar crystalfragmentsoflabradoriteandoli goclase labradorite 40 ar gain diffusion profiles were measured yielding an activation energy of 26 72 4 58 kcal mol 21 118 0 19 16 kj mol 21 and a frequency factor of 9 77 10 29 8 79 10 28 28 79 10 29 c m 2 s 21 95 confidence the ar solubility in labradorite was measured yielding a value of 0 2 ppb bar 21 which is similar to or lower than many rock forming minerals the labradorite diffusion parameters indicate ar closure temperatures of 211 8c for a spherical diffusion geometry and 243 8c for a planar diffusion geometry for 100 mm diameter grains with cooling rates of 10 8 cm a 21 the data indicate that labradorite is less ar retentive than k feldspar at low temperatures but more ar retentive than k feldspar at high temperatures corroborating previous work on plagioclase the relatively slow ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems supplementary material details of the uv laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at www geolsoc org uk sup18608

evidence of rainfall variations in southern brazil from trace element ratios mg ca and sr ca in a late pleistocene stalagmite trace element ratios in the bt2 stalagmite from botuveracave southern brazil are explored as a proxy for changes in the local rainfall recharge during the last 116 ky bp mg ca and sr ca ratios measured with an electron microprobe are signif icantly positively correlated with one another throughout the entire record and vary in a way that is very consistent with variations of d 18 o in the same speleothem during the last glacial period we suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of mg and sr into calcite of the stalagmite this interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting therefore we conclude that higher lower mg ca and sr ca values are associated with lower higher levels of recharge into the karstic aquifer as such conditions lead to an increase decrease in the volume of calcite precipitated in the unsaturated zone above the cave during dry wet climate periods trace element variations point to generally dryer wetter conditions during lower high phases of summer insolation in the southern hemisphere these periods coincide with decreased increased activity of the south american summer monsoon as revealed by d 18 o stalagmite records in addition trace element variations show that rather wet conditions persisted through out most of the last glacial period from approximately 70 to 17 ky bp we suggest that during this period the glacial boundary conditions especially ice volume buildup in the northern hemisphere played an important role for monsoon rainfall inten sification in the region 2007 elsevier ltd all rights reserved

partition of noble gases between olivine and basalt melt abstract the distribution coefficients ki of noble gases were determined for synthesized olivine basalt melt pairs seven samples bh series were synthesized at 1370 1300 c under a one atmospheric pressure 105 pa noble gas mixture and four samples hpp and hpg series at 1360 1050 c at high pressures 0 2 1 5 gpa the two different experiments gave consistent results with each other some bh series olivine samples were apparently contaminated with gas rich inclusions or the glass phase and the effects of such interferences on the calculated distribution coefficients were evaluated the possible ranges of the distribution coefficients of noble gases thus obtained are khe 0 07 kne 0 006 0 08 kar 0 05 0 15 kkr 0 15 and kxe 0 3 these show remarkable features compared to solid element distribution coefficients the distribution coefficients of noble gases are rather insensitive to or even positively correlated with the atomic size of noble gases the striking contrast between the partition of noble gases and that of solid elements may be attributed to the electrical neutrality of noble gases and the increasing electronic polarizability from he to xe

noble gas distribution between basalt melt and crystals we studied the partition of ar and kr between synthetized crystals clinopyroxene olivine and a basalt melt clinopyroxene glass pairs were synthetized in a platinum capsule containing air with a piston cylinder type high pressure apparatus at 1 5 gpa at1250 1175 c olivine glass pairs were synthesized at1350 1300 c in an electric furnace with ar flow distribution coefficients noble gas content in crystal noble gas content in melt measured for these crystal melt pairs are0 2 0 5 for clinopyroxene glass for ar and kr and0 08 0 18 for olivine glass for ar

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

interaction of the south american monsoon system and the southern westerly wind belt during the last 14 kyr abstract surface currents and sediment distribution of the se south american upper continental margin are under the influence of the south american monsoon system sams and the southern westerly wind belt swwb both climatic systems determine the meridional position of the subtropical shelf front stsf and probably also of the brazil malvinas confluence bmc we reconstruct the changing impact of the sams and the swwb on sediment composition at the upper rio grande cone off southern brazil during the last 14 cal kyr bp combining sedimentological geochemical micropaleontological and rock magnetic proxies of marine sediment core geob 6211 2 sharp reciprocal changes in ferri and paramagnetic mineral content and prominent grain size shifts give strong clues to systematic source changes and transport modes of these mostly terrigenous sediments our interpretations support the assumption that the sams over se south america was weaker than today during most of the late glacial and entire early holocene while the swwb was contracted to more southern latitudes resembling modern austral summer like conditions in consequence the stsf and the bmc were driven to more southern positions than today s favoring the deposition of fe rich but weakly magnetic la plata river silts at the rio grande cone during the mid holocene the northern boundary of the swwb migrated northward while the stsf reached its northernmost position of the last 14 cal kyr bp and the bmc most likely arrived at its modern position this shift enabled the transport of antarctic diatoms and more strongly magnetic argentinean shelf sands to the rio grande cone while sediment contributions from the la plata river became less important during the late holocene the modern el nino southern oscillation set in and the sams and the austral tradewinds intensified causing a southward shift of the stsf to its modern position this reinforced a significant deposition of la plata river silts at the rio grande cone these higher magnetic silts with intermediate fe contents mirror the modern more humid terrestrial climatic conditions over se south america

holocene shifts of the subtropical shelf front off southeastern south america controlled by high and low latitude atmospheric forcings over the uruguayan shelf and uppermost slope the coalescence of northward flowing subantarctic shelf water and southward flowing subtropical shelf water forms a distinct thermohaline front termed the subtropical shelf front stsf running in a sw direction diagonally across the shelf from the coastal waters at 32 s toward the shelf break at ca 36 s the stsf represents the shelfward extension of the brazil malvinas confluence zone this study reconstructs latitudinal stsf shifts during the holocene based on benthic foraminifera 18o and 13c total organic carbon carbonate contents ti ca and grain size distribution from a high accumulation sedimentary record located at an uppermost continental slope terrace our data provide direct evidence for 1 a southern stsf position to the south of the core site at the beginning of the early holocene 9 4 cal ka bp linked to a more southerly position of the southern westerly winds in combination with restricted shelf circulation intensity due to lower sea level 2 a gradual stsf northward migration bypassing the core site toward the north primarily forced by the northward migration of the southern westerly winds from 9 4 cal ka bp onward 3 a relatively stable position of the front in the interval between 7 2 and 4 0 cal ka bp 4 millennial scale latitudinal oscillations close to 36 s of the stsf after 4 0 cal ka bp probably linked to the intensification in el nino southern oscillation and 5 a southward migration of the stsf during the last 200 years possibly linked to anthropogenic influences on the atmosphere

timing duration and transitions of the last interglacial asian monsoon thorium 230 ages and oxygen isotope ratios of stalagmites from dongge cave china characterize the asian monsoon and low latitude precipitation over the past 160 000 years numerous abrupt changes in 18o 16o values result from changes in tropical and subtropical precipitation driven by insolation and millennial scale circulation shifts the last interglacial monsoon lasted 9 7 1 1 thousand years beginning with an abrupt less than 200 years drop in 18o 16o values 129 3 0 9 thousand years ago and ending with an abrupt less than 300 years rise in 18o 16o values 119 6 0 6 thousand years ago the start coincides with insolation rise and measures of full interglacial conditions indicating that insolation triggered the final rise to full interglacial conditions

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for