sediment yield exceeds sediment production in arid region drainage basins we use 10 be and 26 al to determine long term sediment generation rates identify significant sediment sources and test for landscape steady state in nahal yael an extensively studied hyperarid drainage basin in southern israel comparing a 33 yr sediment budget with 33 paired 10 be and 26 al analyses indicates that short term sediment yield 113 138 t km 2 yr 1 exceeds long term sediment production 74 16 t km 2 yr 1 by 53 86 the difference suggests that the basin is not in steady state but is currently evacuating sediment accumulated during periods of more rapid sediment generation and lower sediment yield nuclide data indicate that 1 sediment leaving the basin is derived primarily from hillslope colluvium 2 bedrock weathers more rapidly beneath a cover of colluvium than when exposed and 3 long term erosion rates of granite schist and amphibolite are similar

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

paleomagnetic constraints on the crustal evolution of the yangtze block southeastern china abstract paleomagnetic poles for the yangtze block ranging in age from early cambrian to quaternary are compiled up to the end of 1989 the data are classified into a b c or d quality categories by applying a specific reliability grading scale a filtering technique is then used to separate out the less reliable from the more reliable poles unit weight is finally assigned to the remaining results to establish an apw path for this block the new apw path reveals two sharp hairpins in the permian jurassic apw segment which were not observed in previous apw paths the first hairpin occurs around the permian triassic boundary which may be related to the rapid subduction of the qinling ocean beneath the sino korean block the second one occurs around the triassic jurassic boundary corresponding to the late indosinian orogeny in addition the early paleozoic path roughly describes an anticlockwise self closing loop which is opposite in sense to that found in the apw path for gondwana land and implies that the yangtze block had been separated from gondwana land since that time the crustal evolution for the yangtze block is presented in terms of paleolatitude and paleorotation during the paleozoic and the early mesozoic the block was located near the equator after the late permian the block underwent considerable poleward motion until it approached its present relative latitudinal position and orientation in the cretaceous relative motions of the yangtze block with respect to eurasia are discussed by comparing their apw paths showing that the yangtze block was not part of the eurasian continent until the cretaceous and that since then no substantial motions occurred between them

parental magma compositions inferred from the chemical compositions of olivine controlled derivative melts abstract the melt portion of a parental magma undergoing equilibrium olivine crystallization passes through a succession of compositions unique to its initial composition because the succession of compositions follows uniquely from the parental magma s composition a liquid line of descent model fit to several of the derivative melt compositions can recover an estimate of the parental magma s composition we have developed an algorithm to accomplish the model fitting and thus far equipped it to treat olivine fractionation including departures from perfect equilibrium crystallization here we report validation tests on the method which we call derivative magma analysis dma applied to computer synthesized data and two instances of laboratory synthesized data one as a single blind test for the computer and laboratory synthesized data dma recovered the parental magma compositions within common analytical accuracy for the most difficult oxide to estimate mgo the recovered values deviated from the measured values by less than 0 5 wt for the two laboratory synthesized datasets for other major element oxides recovered values typically deviate from nominal values by less than 0 2 wt dma also reliably estimated the compositions of the olivines that coexisted with each derivative melt applied to the kilauean summit eruption of 1959 dma estimates a parental magma with mg 73 mgo 16 wt this value while significantly higher than estimates from some authors who favor low mg parental magmas rules out the extremely high mgo parental magmas favored by other authors

a 130 ka reconstruction of rainfall on the bolivian altiplano new efforts to link climate reconstructions from shoreline deposits and sediment cores yield an improved and more detailed lake history from the bolivian altiplano on the southern altiplano 10 lake oscillations have been identified from this new unified chronology each coincident with north atlantic cold events such as heinrich events h5 h2 h1 and the younger dryas by coupling this new lake history to a hydrologic budget model we are able to evaluate precipitation variability on the southern bolivian altiplano over the last 130 ka these modeling efforts underscore the relative aridity of the altiplano during the rare and small lake cycles occurring between 80 and 20 ka when colder temperatures combined with little or no change in rainfall produced smaller paleolakes relative aridity between 80 and 20 ka contrasts with the immense tauca lake cycle 18 1 14 1 ka which was six times larger than modern lake titicaca and coincided with heinrich event 1 this improved paleolake record from the southern altiplano reveals a strong link between central andean climate and atlantic sea surface temperature gradients during the late pleistocene even though today rainfall variability is driven mostly by pacific sea surface temperature anomalies associated with el nino southern oscillation however not all heinrich events appear to result in lake expansions most conspicuously during the global cold interval between 80 and 20 ka when the altiplano and amazon basin were relatively arid

gisp2 oxygen isotope ratios the gisp2 oxygen isotope record with its high resolution detail yields crucial information on past climate change the glacial 18o oscillations of the gisp2 core with their very fast onsets are templates of a prototype oscillation of variable duration with an amplitude of 3 9 the halfway mark of the cold warm transition is reached in 2 years the top is reached in 50 years the 18o time gradient of the leading front is about 7 8 per 100 yr after reaching the top 18o slowly declines by 0 14 per 100 yr the duration of 18o decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones the subsequent 18o downturn during the warm cold transition has a 18o time gradient of 3 2 per 100 yr and lasts about 80 yr

a geologically constrained monte carlo approach to modeling exposure ages from profiles of cosmogenic nuclides an example from lees ferry arizona we present a user friendly and versatile monte carlo simulator for modeling profiles of in situ terrestrial cosmogenic nuclides tcns our program available online at http geochronology earthsciences dal ca downloads models html permits the incorporation of site specific geologic knowledge to calculate most probable values for exposure age erosion rate and inherited nuclide concentration while providing a rigorous treatment of their uncertainties the simulator is demonstrated with 10be data from a fluvial terrace at lees ferry arizona interpreted constraints on erosion based on local soil properties and terrace morphology yield a most probable exposure age and inheritance of 83 9 14 1 19 1 ka and 9 49 2 52 1 21 104 atoms g 1 respectively 2 without the ability to apply some constraint to either erosion rate or age shallow depth profiles of any cosmogenic nuclide except for nuclides produced via thermal and epithermal neutron capture e g 36cl cannot be optimized to resolve either parameter contrasting simulations of 10be data from both sand and pebble sized clasts within the same deposit indicate grain size can significantly affect the ability to model ages with tcn depth profiles and when possible sand not pebbles should be used for depth profile exposure dating

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

high level surfaces plateau uplift and foreland development bolivian central andes newly dated tertiary strata in the bolivian central andean plateau and synthesis of the tertiary record in the adjacent subandean fold thrust belt constrain the age of deformation in both regions age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high level surface known as the san juan del oro surface new 40 ar 39 ar dates on undeformed strata above the high level surface preclude significant upper crustal shortening within the eastern cordillera after 10 ma tertiary strata within the adjacent subandean region demonstrate that formation of the fold thrust belt occurred after 10 ma on the basis of these data we propose a two stage model of late cenozoic andean growth that links plateau uplift to the development of the fold thrust belt in the first stage early plateau uplift occurred in response to widespread compressional deformation of the plateau eastern cordillera and altiplano during the second stage beginning after 10 ma upper crustal deformation within the plateau terminated and the subandean fold thrust belt developed crustal scale eastward thrusting along the eastern margin of the eastern cordillera drove subandean folding and thrusting the eastern cordillera served as the bulldozer for the deforming subandean wedge

quaternary glaciation and hydrologic variation in the south american tropics as reconstructed from the lake titicaca drilling project a 136 m long drill core of sediments was recovered from tropical high altitude lake titicaca bolivia peru enabling a reconstruction of past climate that spans four cycles of regional glacial advance and retreat and that is estimated to extend continuously over the last 370 000 yr within the errors of the age model the periods of regional glacial advance and retreat are concordant respectively with global glacial and interglacial stages periods of ice advance in the southern tropical andes generally were periods of positive water balance as evidenced by deeper and fresher conditions in lake titicaca conversely reduced glaciation occurred during periods of negative water balance and shallow closed basin conditions in the lake the apparent coincidence of positive water balance of lake titicaca and glacial growth in the adjacent andes with northern hemisphere ice sheet expansion implies that regional water balance and glacial mass balance are strongly influenced by global scale temperature changes as well as by precessional forcing of the south american summer monsoon

geomorphic disequilibrium in the eastern korean peninsula possible evidence for reactivation of a rift flank margin abstract tectonically inactive since the middle miocene the korean peninsula is generally considered a typical passive continental margin with asymmetric relief with only low levels of seismic activity recent reports of uplift rates as high as 150 350 mm ka 1 from geomorphic markers using numerical dating methods e g radiocarbon optically stimulated luminescence osl and cosmogenic nuclides however cast doubt on the tectonic quiescence of korea thus we evaluate the geomorphic status of eastern korea using denudation rates at geomorphic timescales of 103 106 years which are equivalent to the rates previously reported at local points to infer regional denudation rates we measured catchment wide denudation rates cwdr in 13 watersheds using in situ cosmogenic 10be and 14c analyses of riverine sediment samples in eastern korea these cwdr data suggest that the mean denudation rate during the past 5 14 ka centers around 74 7 25 4 mm ka 1 which is consistent with the long term geologic exhumation rate we derive of 74 10 1 mm ka 1 since the middle miocene however our cwdr data are 2 3 times lower than rates of coastal uplift and river incision of 150 350 mm ka 1 derived from data on marine and fluvial terraces respectively in addition to isotopic analyses we quantified topographic indices such as elevation relief and slope and then compared these indices with cwdrs to examine morphological changes that might be related to tectonic disturbances since the beginning of late pleistocene the relationship between cwdrs and topographic indices indicates that most of the study area rests in a near steady state condition however in contrast to the smooth concave longitudinal profile expected for a steady state landscape topographic analyses reveal the presence of major knickzones confirmed by field reconnaissance these knickzones may have been produced by changes in base level caused by tectonic events lithologic variations or sea level changes our 1 d modeling of knickpoint propagation suggests that tectonic disturbance is the factor most likely to produce knickzones in the study area raising the possibility that erosional signals caused by late pleistocene tectonic disturbance affected channel gradients but that the knickpoints have not yet propagated through the entire catchment area our results suggest that the eastern part of the korean peninsula is approaching but has not completely reached a geomorphic steady state and that transient conditions have been induced by reactivation of the rift flank margin no later than the late pleistocene causing tilting of the central korean peninsula to the west at a rate of 7 0 10 10 m m per year over the past 125 ka

paleomagnetism and question of original location of the permian brook street terrane new zealand over 400 rock samples from 30 sites were collected for paleomagnetic study from the volcanogenic section in the brook street terrane within the takitimu mountains in western southland new zealand the section includes igneous and sedimentary rocks of the permian takitimu group and white hill intrusive suite many of the samples show a partial or complete remagnetization in the present field because of a recent acquisition of viscous remanent magnetization an early permian direction inclination 46 1 declination 257 isolated from the heartbreak and chimney peaks formations of the takitimu group indicates a low paleolatitude to midpaleolatitude position 27 5 for the brook street terrane directions from the late permian white hill intrusives inclination 64 6 declination 173 3 suggest a slightly higher paleolatitude than the early permian takitimu group directions and 70 90 of intervening clockwise rotation plate reconstructions and paleomagnetic data predict a high paleolatitude for the new zealand margin of gondwana throughout the late paleozoic and mesozoic the low paleolatitude to middle paleolatitude implied by the early permian brook street result together with the oceanic nature of the brook street arc suggest that the brook street terrane is allochthonous to the margin of gondwana a published late triassic early jurassic paleomagnetic pole from the adjacent murihiku terrane indicates a high paleolatitude this suggests that the brook street and murihiku terranes are genetically distinct

debris flow dependent variation of cosmogenically derived catchment wide denudation rates catchment wide denudation rates cwdrs obtained from cosmogenic nuclides are an efficient way to determine geomorphic processes quantitatively in alpine mountain ranges over holocene time scales these rate estimations assume steady geomorphic processes here we use a time series 3 yr in the aare catchment central swiss alps to test the impact of spatially heterogeneous stochastic sediment supply on cwdrs our results show that low frequency high magnitude debris flow events significantly perturb cosmogenic nuclide 10 be 14 c concentrations and thus cwdrs the 10 be concentrations decrease by a factor of two following debris flow events resulting in a doubling of inferred cwdrs the variability indicates a clear time and source dependency on sediment supply with restricted area weighted mixing of sediment accordingly in transient environments it is critical to have an understanding of the history of geomorphic processes to derive meaningful cwdrs we hypothesize that the size of debris flows their connectivity with the trunk stream and the ability of the system to sufficiently mix sediment from low and high order catchments control the magnitude of cwdr perturbations we also determined in situ 14 c in a few samples in conjunction with 10 be these data suggest partial storage for colluvium of a few thousand years within the catchment prior to debris flow initiation

absolute reconstructions of the paleozoic oceans abstract the orientation of three intraplate magmatic belts the mongolian from 130 to 280 m y south siberian from 320 to 400 m y and baltic from 365 to 400 m y belts which can be considered as hot spot tracks are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during paleozoic times the reconstructions obtained show that the old late precambrian pangea continued to exist in the early paleozoic with an inner arrangement of constituent continents strongly different from that of the late paleozoic pangea the continents were constantly located in the eastern hemisphere only the western hemisphere was occupied by the paleo pacific oceans four oceans existed in paleozoic times between the continents in the eastern hemisphere 1 the iapetus which originated in the late precambrian and closed before devonian times 400 m y 2 the asiatic paleo ocean which originated in the late precambrian and closed 450 m y ago 3 the uralian paleo ocean which opened around 500 m y ago and closed in the latest permian 240 230 m y 4 the paleo tethys which opened in the ordovician 480 450 m y and became the mesozoic tethys life duration of the oceans was 200 400 m y the main trend in the earth s evolution during the paleozoic was a break up of the old precambrian supercontinent and construction of the new late paleozoic pangea

the climate of the altiplano observed current conditions and mechanisms of past changes abstract the large scale controls on the climate of the south american altiplano are investigated using local observations reanalysis data and general circulation model experiments the objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial interglacial our results suggest that on all timescales the climatic conditions on the altiplano are closely related to the upper air circulation with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions different factors influence the upper air circulation on the different timescales intraseasonal variability is a reflection of the position and intensity of the bolivian high which is modulated by rossby waves emanating from the midlatitude south pacific the annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere rather than direct insolation forcing over the altiplano or moisture changes in the source area interannual variability is primarily related to changes in the mean zonal flow over the altiplano reflecting changes in meridional baroclinicity between tropical and subtropical latitudes which in turn is a response to sea surface temperature changes in the tropical pacific orbitally forced changes in the land sea contrast drive continental scale circulation changes which significantly alter the zonal flow over the altiplano on glacial interglacial timescales the contrast in heating between northern and southern hemispheres during the glacial leads to upper air easterly anomalies throughout the tropics on modern timescales the marked submonthly seasonal and interannual changes of moisture over the altiplano cannot be accounted for by moisture changes in the humid tropical lowlands however the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands which counteracts the effect of the upper level circulation resulting in little overall change in precipitation this observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the altiplano on a wide range of timescales

das phanomen trias kurze ubersicht uber palaogeographische palaotektonische und palaoklimatologische probleme der trias unter ausklammerung des germanischen beckens allgemeiner rahmen zu den referaten der trias tagung 1982 in wurzburg

the ad 1717 rock avalanche deposits in the upper ferret valley italy a dating approach with cosmogenic 10be on 12 september ad 1717 a rock volume larger than 10 million m3 collapsed onto the triolet glacier mobilized a mass composed of ice and sediment and travelled more than 7 km downvalley in the upper ferret valley mont blanc massif italy this rock avalanche destroyed two small settlements causing seven casualties and loss of livestock no detailed maps were made at the time later investigators attributed accumulations of granitic boulders and irregular ridges on the upper valley floor to either glacial deposition or the ad 1717 rock avalanche or a complex mixture of glacial deposition earlier rock avalanche and ad 1717 rock avalanche origin in this study we present cosmogenic 10be exposure ages from nine boulders in the extensive chaotic boulder deposit with irregular ridges two from holocene glacier free areas and one from a little ice age moraine exposure ages between 330 23 and 483 123 a from eight of nine boulders from the chaotic deposit indicate that at least seven were deposited by the ad 1717 rock avalanche the other three boulders yielded 10be exposure ages of 10 900 400 9700 400 and 244 97 a respectively our results are in good agreement with the existing chronology from dendrochronology and lichenometry and radiocarbon analysis of wood samples but not with older 14c ages from a peat bog in the upper part of the valley based on the new age control the rock avalanche deposits cover the whole bottom of the upper ferret valley copyright 2012 john wiley sons ltd

rock uplift rates in south africa from isochron burial dating of fluvial and marine terraces the high plateau of southern africa is considered to be either uplifting due to mantle driven dynamic topography or to have been stable since mesozoic rifting to address this debate we determined rock uplift in south africa from the long term incision rate of the sundays river near port elizabeth and from an uplifted marine terrace near durban we dated the terraces with cosmogenic 26 al and 10 be using both isochron and simple burial dating methods we find that the sundays river has incised at 16 1 1 3 m m y for the past 4 m y and the marine terrace yields a rock uplift rate of 9 4 2 2 m m y these results are inconsistent with rapid neogene uplift

early crystallization history of alkali olivine basalts diamond craters oregon abstract basaltic lavas form the isolated quaternary volcanic centers of diamond craters and occur at cow lakes in s e oregon the exposed rocks are olivine and plagioclase phyric alkali olivine basalts the holocrystalline groundmasses contain plagioclase titanaugite olivine fe ti oxides and minor apatite and occur in the high alumina greater than 17 petrographic province of the northwestern great basin the whole rock and phenocryst compositions provide enough information to define the equilibrium state of the magmas the solutions to the thermodynamic equations are valid for systems of known bulk composition and can be used to evaluate whether rock compositions are close to magma compositions two of the lavas from diamond craters have whole rock and phenocryst compositions consistent with their being natural melts rock compositions that reflect the effects of crystal separation open systems are recognized by significant differences in predicted versus observed phenocryst modes the p t conditions of phenocryst saturation and the crystallization history of the magmas can be inferred from petrologic data combined with thermodynamic modelling and mass balance constraints the initial melts are inferred to have equilibrated at 0 8 1 0 gpa and 1210 1240 c the majority of the lavas from diamond craters have compositions that reflect olivine and plagioclase crystal separation these lavas can be related to two possible parent rock compositions by small amounts 2 8 by weight of olivine and plagioclase separation

steady state reach scale theory for radioactive tracer concentration in a simple channel floodplain system 1 a steady state analytical model is presented for reach scale variation in the concentration of a decaying radioactive tracer associated with sediment particles that regularly pass through an off channel floodplain the floodplain is represented as a series of well mixed sediment reservoirs that continually exchange sediment with the channel the model allows for tributary input and valley wide aggradation or degradation tracer concentration depends on the upstream boundary concentration the tracer and sediment load floodplain geometry and the rates of in floodplain tracer production and or decay the theory predicts relatively modest down channel change in the concentration of long lived isotopes but implies that significant change may occur for 1 tracers with a short enough half life such as 14c or 2 floodplains with sediment residence times that are large enough for cosmogenic production or meteoric fallout to increase tracer concentration in the down valley direction the profiles are shown to be strongly dependent on the grain size distributions of both the sediment load and the floodplain the results imply that down channel 14c profiles have the potential to constrain holocene bed material loads in systems with sufficient storage the theory concisely describes the general importance of a floodplain for modifying in situ produced cosmogenic tracer concentration and can also characterize floodplain importance for fallout radioisotopes i e 10be 210pb or 7be or organic 14c

ice core palaeoclimate records in tropical south america since the last glacial maximum ice core records spanning the last 25 000 yr from the tropical andes of south america are reviewed these records from quelccaya huascaran and sajama present a high temporal resolution picture of both the late glacial stage lgs and the holocene climatic and environmental conditions in the south american andes late glacial stage conditions at high elevations appear to have been cooler than today although the magnitude of the inferred cooling differs with the particular proxy used e g snowline depression pollen ice cores insoluble dust and anion concentrations in the ice cores reveal that lgs hydrological conditions in the tropics 9 s were much drier than today whereas in the subtropics 18 s lgs conditions were much wetter this probably reflects the migration of the tropical hadley cell in response to a different meridional temperature gradient low nitrate concentrations in the lgs ice from both huascaran and sajama suggest that the amazon basin forest cover may have been much less extensive discussed is the conundrum surrounding the use of d 18 o as a palaeothermometer in the tropics where temperatures exhibit little seasonal variation yet the ice core records suggest that d 18 o records temperature variations on decadal to millennial time scales finally evidence is presented for a strong twentieth century warming copyright 2000 john wiley sons ltd

a new andean deep ice core from nevado illimani 6350 m bolivia abstract a new ice core record from the nevado illimani 16 s bolivia covers approximately the last 18 000 years bp a comparison with two published ice records from sajama 18 s bolivia thompson et al science 282 1998 1858 1864 and huascaran 9 s peru thompson et al science 269 1996 46 50 documents a regionally coherent transition from glacial to modern climate conditions in south america north of 20 s the strong resemblance between the illimani and huascaran water isotope records and their differences from the sajama record in particular during the period from 9000 years bp to 14 000 years bp suggest that local water recycling or local circulation changes played a major role for sajama we interpret the common illimani huascaran water isotope history in terms of a common change from wetter cooler conditions during glacial times to drier warmer conditions in the early holocene

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

endmember unmixing of compositional data abstract manipulation of the n endmember mixing equations appropriate to compositional data leads to a general method 1 to test hypotheses about endmember compositions and 2 to recover the concentrations of all compositional components in an endmember when the concentrations of n 1 components are known or measured in that endmember additionally pearce element ratio analysis is shown to be a special and not efficiently posed two endmember case of n endmember unmixing information available on pearce element ratio diagrams as values of the slopes of lines is contained on harker diagrams as values of points

combined slope ratio analysis and linear subtraction an extension of the pearce ratio method abstract a new technique called combined slope ratio analysis has been developed by extending the pearce element ratio or conserved denominator method pearce 1968 to its logical conclusions if two stoichiometric substances are mixed and certain chemical components are uniquely contained in either one of the two mixing substances then by treating these unique components as conserved the composition of the substance not containing the relevant component can be accurately calculated within the limits allowed by analytical and geological error the calculated composition can then be subjected to rigorous statistical testing using the linear subtraction method recently advanced by woronow 1994 application of combined slope ratio analysis to the rocks of the uwekahuna laccolith hawaii usa and the lavas of the 1959 summit eruption of kilauea volcano hawaii usa yields results that are consistent with field observations

cosmogenic nuclide budgeting of floodplain sediment transfer abstract cosmogenic nuclides produced in quartz may either decay or accumulate while sediment is moved through a river basin a change in nuclide concentration resulting from storage in a floodplain is potentially important in large drainage basins in which sediment is prone to repeated burial and remobilization as a river migrates through its floodplain we have modeled depth and time dependent cosmogenic nuclide concentration changes for 10 be 26 al and 14 c during sediment storage and mixing in various active floodplain settings ranging from confined shallow rivers with small floodplains to foreland basin scale floodplains traversed by deep rivers floodplain storage time estimated from channel migration rates ranges from 0 4 kyr for the beni river basin bolivia to 7 kyr for the amazon river basin while floodplain storage depth estimated from channel depth ranges from 1 to 25 m for all modeled active floodplain settings the long lived nuclides 10 be and 26 al show neither significant increase in nuclide concentration from irradiation nor decrease from decay we predict a hypothetical response time after which changes in 10 be or 26 al concentrations become analytically resolvable this interval ranges from 0 07 to 2 myr and exceeds in all cases the typical residence time of sediment in a floodplain due to the much shorter half life of 14 c nuclide concentrations modeled for the in situ produced variety of this nuclide are however sensitive to floodplain storage on residence times of the cosmogenic nuclide composition of old deposits in currently inactive floodplains that have been isolated for periods of millions of years from the river that once deposited them is predicted to either increase or decrease in 10 be and 26 al concentration depending on the depositional depth these conditions can be evaluated using the 26 al 10 be ratio that readily discloses the depth and duration of storage we illustrate these models with examples from the amazon basin as predicted modern bedload collected from an amazon tributary the bolivian beni river shows no systematic change in nuclide concentration as sediment is moved through 500 km of floodplain by river meandering in contrast in the central amazon floodplain currently untouched by the modern river system low 26 al 10 be ratios account for minimum burial depths of 5 to 10 m for a duration of 5 myr the important result of this analysis is that in all likely cases of active floodplains cosmogenic 10 be and 26 al concentrations remain virtually unchanged over the interval sediment usually spends in the basin thus spatially averaged denudation rates of the sediment producing area can be inferred throughout the entire basin provided that nuclide production rates are scaled for the altitudes of the sediment producing area only because floodplain storage does not modify nuclide concentrations introduced from the sediment source area

possible flexural accommodation on the eastern edge of the altiplano in relation to focussed erosion in the rio la paz drainage system it has been proposed that erosional unloading by focused incision can be accommodated by flexural rebound which in turn potentially exerts a positive feedback on erosion this paper presents morphometric data from the la paz drainage basin in the bolivian andes to illustrate the effects of such a feedback mechanism between erosion and crustal bending the rio la paz is a prominent system with the headwaters on the altiplano and a trunk stream segment that cuts across the cordillera real thereby connecting large portions of the altiplano with the amazon basin the flexural feedback model explains why all drainages beyond the watershed disperse their waters to the altiplano it also provides an explanation for the presence of the highest peaks just next to the location where the la paz river cuts into the bedrock across the cordillera real

cosmogenic dating of fluvial terraces fremont river utah absolute dating of river terraces can yield long term incision rates clarify the role of climate in setting times of aggradation and incision and establish the rates of pedogenic processes while surface exposure dating using cosmogenic 10be and 26al would seem to be an ideal dating method the surfaces are composed of individual clasts each with its own complex history of exposure and burial the stochastic nature of burial depth and hence in nuclide production in these clasts during exhumation and fluvial transport and during post depositional stirring results in great variability in clast nuclide concentrations we present a method for dealing with the problem of pre depositional inheritance of cosmogenic nuclides we generate samples by amalgamating many individual clasts in order to average over their widely different exposure histories depth profiles of such amalgamated samples allow us to constrain the mean inheritance to test for the possible importance of stirring and to estimate the age of the surface working with samples from terraces of the fremont river we demonstrate that samples amalgamated from 30 clasts represent well the mean concentration depth profiles show the expected shifted exponential concentration profile that we attribute to the sum of uniform mean inheritance and depth dependent post depositional nuclide production that the depth dependent parts of the profiles are exponential argues against significant post depositional displacement of clasts within the deposit our technique yields 10be age estimates of 60 9 102 16 and 151 24 ka for the three highest terraces corresponding to isotope stages 4 5d and 6 respectively the mean inheritance is similar from terrace to terrace and would correspond to an error of 30 40 ka if not taken into account the inheritance likely reflects primarily the mean exhumation rates in the headwaters of order 30 m ma

fluvial dynamics and 14c 10be disequilibrium on the bolivian altiplano determining sediment transfer times is key to understanding source to sink dynamics and the transmission of environmental signals through the fluvial system previous work on the bolivian altiplano applied the in situ cosmogenic 14c 10be chronometer to river sands and proposed sediment storage times of 10 20 kyr in four catchments southeast of lake titicaca however the fidelity of those results hinges upon isotopic steady state within sediment supplied from the source area with the aim of independently quantifying sediment storage times and testing the 14c 10be steady state assumption we dated sediment storage units within one of the previously investigated catchments using radiocarbon dating cosmogenic 10be 26al isochron burial dating and 10be 26al depth profile dating palaeosurfaces appear to preserve remnants of a former fluvial system which has undergone drainage reversal reduction in catchment area and local isostatic uplift since 2 8 ma from alluvium mantling the palaeosurfaces we gained a deposition age of 580 ka while lower down fluvial terraces yielded 34 ka and floodplains 3 1 ka owing to restricted channel connectivity with the terraces and palaeosurfaces the main source of channel sediment is via reworking of the late holocene floodplain yet modelling a set of feasible scenarios reveals that floodplain storage and burial depth are incompatible with the 14c 10be disequilibrium measured in the channel instead we propose that the 14c 10be offset results from i non uniform erosion whereby deep gullies supply hillslope derived debris and or ii holocene landscape transience associated with climate or human impact the reliability of the 14c 10be chronometer vitally depends upon careful evaluation of sources of isotopic disequilibrium in a wide range of depositional and erosional landforms in the landscape

exhumation history of the southern altiplano plateau southern bolivia constrained by apatite fission track thermochronology 1 although the structural geometry of the cenozoic altiplano puna plateau in the central andes is well defined the temporal evolution of this contractile deformation is poorly constrained to address this shortcoming we used apatite fission track thermochronology aft to quantify the cooling and exhumation history along a transect at 21 s in southern bolivia through the deformed intermontane altiplano basin the doubly vergent thrust belt of the eastern cordillera and the inner foreland thrust belt east of the plateau interandean zone thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories exhumation started during the late eocene 40 36 ma in the central eastern cordillera possibly due to bivergent thrusting and cretaceous rift structure inversion during the early oligocene 33 27 ma exhumation spread across the study area as the current boundary thrusts of the eastern cordillera were activated the inner west vergent thrust system became active in irregular order until circa 20 ma whereas the east vergent interandean thrust belt formed by eastward propagating deformation since circa 30 ma plateau exhumation continued at 0 2 mm yr until shortening terminated by 11 7 ma shortening within the plateau since circa 30 ma did not evolve by lateral accretion of thrust wedges the propagating deformation style is spatially confined to the foreland thrust belt which initiated coeval to plateau deformation interandean zone but propagated mainly after circa 10 ma subandean zone early oligocene plateau wide tectonically driven exhumation suggests that subduction related processes had already thermally weakened the continental lithosphere prior to the 27 25 ma onset of volcanic activity

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

tertiary provenance history of the northern and central altiplano central andes bolivia a detrital record of plateau margin tectonics abstract the internally drained altiplano plateau in the high elevation hinterland of the central andes contains an extremely thick up to 12 km succession of tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins in this study temporal and spatial variations in detrital composition based on 113 sandstone thin sections and 31 conglomerate clast counts combined with sediment dispersal patterns and regional stratigraphic correlations help delineate the tertiary history of altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins stratigraphic and provenance data suggest limited subsidence and west directed sediment dispersal during the early tertiary history of the altiplano region by late eocene time development of a rapidly subsiding basin probable foredeep was underway as recorded by extensive east directed fluvial systems of the 3 6 5 km thick potoco formation potoco compositional data are compatible with sediment sources in the incipient western cordillera and regions farther west including 1 a probable thrust belt source terrane composed of paleozoic mesozoic strata and precambrian granitic gneissic basement and 2 a subordinate magmatic arc source terrane composed of tertiary volcanic rocks increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence due to either enhanced volcanism or eastward arc migration by late oligocene time west directed fluvial and alluvial fan systems of the upper potoco formation and the 3 km thick coniri formation reached the altiplano basin providing a minimum uplift age for the backthrust belt along the western flank of the eastern cordillera paleozoic clast conglomerate and relatively quartz rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly paleozoic strata in the backthrust belt continued late oligocene to quaternary accumulation in the altiplano reveals that the basin persisted as a depositional region during a well documented period of neogene shortening and uplift in the andean thrust belt farther east this intra orogenic hinterland deposition between eastern and western highs during ongoing shortening suggests that the altiplano basin existed as a crustal scale piggyback basin possibly in an internally drained plateau like geomorphic setting since 25 ma provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the altiplano although flexural subsidence due to thrust loading along both plateau margins initially in the west and subsequently in the east may explain most tertiary subsidence possible closed basin conditions due to structural damming could have promoted additional sediment accumulation from late oligocene to quaternary time

a robust least squares gondwanan apparent polar wander path and the question of palaeomagnetic assessment of gondwanan reconstruction abstract two hundred and nineteen palaeomagnetic results are combined in calculating gondwanan apparent polar wander paths their confidence intervals and the residual mean square errors about the paths for six reconstructions a quantitative assessment of the six reconstructions of gondwanaland is made on the basis of the consistency of palaeomagnetic pole positions using a new statistical technique smith and hallam s 1 gondwanan reconstruction is the most effective in accounting for the distribution of the palaeomagnetic data and is used in calculating our preferred apparent polar wander path

detection of transience in eroding landscapes past variations in climate and tectonics have led to spatially and temporally varying erosion rates across many landscapes in this contribution i examine methods for detecting and quantifying the nature and timing of transience in eroding landscapes at a single location cosmogenic nuclides can detect the instantaneous removal of material or acceleration of erosion rates over millennial timescales using paired nuclides detection is possible only if one of the nuclides has a significantly shorter half life than the other currently the only practical way of doing this is to use cosmogenic in situ carbon 14 14c alongside a longer lived nuclide such as beryllium 10 10be hillslope information can complement or be used in lieu of cosmogenic information in soil mantled landscapes increased erosion rates can be detected for millennia after the increase by comparing relief and ridgetop curvature this technique will work as long as the final erosion rate is greater than twice the initial rate on a landscape scale transience may be detected based upon disequilibria in channel profiles or ridgetops but transience can be sensitive to the nature of transient forcing where forcing is periodic landscapes display differing behavior if forcing is driven by changes in base level lowering rates versus changes in the efficiency of either channel or hillslope erosion e g driven by climate change oscillations in base level lowering lead to basin averaged erosion rates that reflect a long term average erosion rate despite strong spatial heterogeneity in local erosion rates this averaging is reflected in 10be concentrations in stream sediments changes in hillslope sediment transport coefficients can lead to large fluctuations in basin averaged erosion rates which again are reflected in 10be concentrations the variability of erosion rates in landscapes where both the sediment transport and channel erodibility coefficients vary is dominated by changes to the hillslope transport coefficient copyright 2016 john wiley sons ltd

geochronology and stratigraphy of late pleistocene lake cycles on the southern bolivian altiplano implications for causes of tropical climate change large paleolakes 33 000 60 000 km2 that once occupied the high altitude poopo coipasa and uyuni basins in southern bolivia 18 22 s provide evidence of major changes in low latitude moisture in these now dry or oligosaline basins extensive natural exposure reveals evidence for two deep lake and several minor lake cycles over the past 120 k y fifty three new u th and 87 new 14c dates provide a chronologic framework for changes in lake level deposits from the ouki deep lake cycle are extensively exposed in the poopo basin but no deep lakes are apparent in the record between 98 and 18 1 ka the ouki lake cycle was 80 m deep and nineteen u th dates place this deep lake cycle between 120 and 98 ka shallow lakes were present in the terminal uyuni basin between 95 and 80 ka salinas lake cycle at ca 46 inca huasi lake cycle and between 24 and 20 5 ka sajsi lake cycle the tauca deep lake cycle occurred between 18 1 and 14 1 ka resulting in the deepest 140 m and largest lake in the basin over the past 120 ka multiple 14c and u th dates constrain the highest stand of lake tauca along a topographically conspicuous shoreline between 16 4 and 14 1 ka a probable post tauca lake cycle the coipasa produced a 55 m deep lake that is tentatively dated between 13 and 11 ka we suggest that paleolakes on the bolivian altiplano expanded in response to increased moisture in the amazon and enhanced transport of that moisture onto the altiplano by strengthened trade winds or southward displacement of the intertropical convergence zone itcz pole to equator sea surface temperature sst and atmospheric gradients may have influenced the position of the itcz affecting moisture balance over the altiplano and at other locations in the amazon basin links between the position of the itcz and the ca 23 ka precessional solar cycle have been postulated march insolation over the altiplano is a relatively good fit to our lake record but no single season or latitude of solar cycling has yet to emerge as the primary driver of climate over the entire amazon basin temperature may influence altiplano lake levels indirectly as potentially dry glacial periods in the amazon basin are linked to dry conditions on the altiplano intensification of the trade winds associated with la nina like conditions currently brings increased precipitation on the altiplano and deep lake development during the tauca lake cycle coincided with apparently intense and persistent la nina like conditions in the central pacific this suggests that sst gradients in the pacific are also a major influence on deep lake development on the altiplano

sediment storage and evacuation in headwater valleys at the transition between debris flow and fluvial processes sediment from landscape disturbance often enters temporary storage in valleys and evacuates over longer times which in steeplands are poorly delimited we hypothesize that across process transitions e g debris fl ow versus fl uvial transport distributions of sediment transit times also change we use fi eld surveys and extensive radiocarbon dating to assess the distribution of transit residence times through the proxy measurement of ages of bank deposits in two mainstem reaches of a 2 23 km 2 watershed in the oregon coast range in the downstream reach debris fans impound fl uvial deposits debris fl ow fi ne fl uvial and coarse fl uvial deposits compose nearly equal parts of the valley fi ll and fl uvial erosion evacuates deposits transit times have a sample mean of 1 22 10 3 14 c yr and an exponential distribution indicating uniform probability of evacuation from storage in the upstream reach valleyspanning debris jams impound debris fl ow deposits composing 95 of the valley fi ll which is routinely scoured by debris fl ows transit times have a sample mean of 4 43 10 2 14 c yr and if 100 14 c yr a power law distribution indicating preferential evacuation of younger deposits and retention of older deposits in both reaches most sediment has short transit times 600 14 c yr but signifi cant volumes remain for millennia less than 20 of basin wide denudation passes through these reservoirs but the latter are still signifi cant buffers between hillslope disturbance and downstream aquatic habitat especially for coarse sediment

the history of south american tropical precipitation for the past 25 000 years long sediment cores recovered from the deep portions of lake titicaca are used to reconstruct the precipitation history of tropical south america for the past 25 000 years lake titicaca was a deep fresh and continuously overflowing lake during the last glacial stage from before 25 000 to 15 000 calibrated years before the present cal yr b p signifying that during the last glacial maximum lgm the altiplano of bolivia and peru and much of the amazon basin were wetter than today the lgm in this part of the andes is dated at 21 000 cal yr b p approximately coincident with the global lgm maximum aridity and lowest lake level occurred in the early and middle holocene 8000 to 5500 cal yr b p during a time of low summer insolation today rising levels of lake titicaca and wet conditions in amazonia are correlated with anomalously cold sea surface temperatures in the northern equatorial atlantic likewise during the deglacial and holocene periods there were several millennial scale wet phases on the altiplano and in amazonia that coincided with anomalously cold periods in the equatorial and high latitude north atlantic such as the younger dryas

timing of european fluvial terrace formation and incision rates constrained by cosmogenic nuclide dating abstract age constraints of late cenozoic fluvial terraces are important for addressing surface process questions related to the incision rates of rivers or tectonic and climate controls on denudation and sedimentation unfortunately absolute age constraints of fluvial terraces are not always possible and many previous studies have often dated terraces with relative age constraints that do not allow for robust interpretations of incision rates and timing of terrace formation however in situ produced cosmogenic nuclides allow absolute age determination and hence incision rates of fluvial deposits back to 5 ma here we present cosmogenic depth profile dating and isochron burial dating of four different river systems in europe spanning 12 of latitude we do this to determine river incision rates and spatial variations in the timing of terrace formation isochron burial age constraints of four selected terraces from the vltava river czech republic range between 1 00 0 21 to 1 99 0 45 ma an isochron burial age derived for the allier river central france is 2 00 0 17 ma five terrace levels from the esla river nw spain were dated between 0 08 0 04 0 01 ma and 0 59 0 13 0 20 ma with depth profile dating the latter age agrees with an isochron burial age of 0 52 0 20 ma two terrace levels from the guadalquivir river sw spain were dated by depth profile dating to 0 09 0 03 0 02 ma and 0 09 0 04 0 03 ma the one terrace level from the guadalquivir river dated by isochron burial dating resulted in an age of 1 79 0 18 ma results indicate that the cosmogenic nuclide based ages are generally older than ages derived from previous relative age constraints leading to a factor 2 3 lower incision rates than previous work furthermore the timing of terrace formation over this latitudinal range is somewhat obscured by uncertainties associated with dating older terraces and not clearly synchronous with global climate variations

hillslope soils and vegetation abstract assessing how vegetation controls hillslope soil processes is a challenging problem as few abiotic landscapes exist as observational controls here we identify five avenues to examine how actively eroding hillslope soils and processes would differ without vegetation and we explore some potential feedbacks that may result in landscape resilience on vegetated hillslopes the various approaches suggest that a plant free world would be characterized by largely soil free hillslopes that plants may control the maximum thickness of soils on slopes that vegetated landforms erode at rates about one order of magnitude faster than plant free outcrops in comparable settings and that vegetated hillslope soils generally maintain long residence times such that both n and p sufficiency for ecosystems is the norm we conclude that quantitatively parameterizing biota within process based hillslope models needs to be a priority in order to project how human activity may further impact the soil mantle

late quaternary spatio temporal limnological variations in the altiplano of bolivia and peru abstract between the western and eastern andean cordilleras in peru and bolivia there are three main lacustrine basins lake titicaca lake poopo and the group of coipasa uyuni for the past few millennia highly variable environmental conditions have been recorded in their sediments today a latitudinal meteorological gradient influences the lakes status leading to specific deposits and ostracod communities lake titicaca in the north is oligohaline whereas lake poopo further south is polyhaline in the south the coipasa uyuni depression is characterized by a 12 000 km2 surficial salt crust during the late pleistocene ca 40 000 to 25 000 yr b p the water depth and salinity in paleolake poopo fluctuated widely and paleolake titicaca was slightly larger than at present sedimentation was mostly biocarbonate in the shallower areas and it was detrital organic in the deepest zones during the holocene a dry period transformed lake poopo into a salar with evaporite precipitation lake titicaca registered a large decline in water level 8100 3600 yr b p initially inducing gypsum precipitation followed by short influxes of water with an ostracod faunal composition similar to that of the modern brines of lake poopo lake titicacas present condition only appeared between 2200 and 1500 yr b p

the nature origin and accumulation of loess abstract loess can be defined simply as a terrestrial clastic sediment composed predominantly of silt size particles which is formed essentially by the accumulation of wind blown dust there are three fundamental requirements for its formation 1 a sustained source of dust 2 adequate wind energy to transport the dust and 3 a suitable accumulation site most loess has been modified to some degree by local reworking bioturbation and syn depositional weathering but a process of loessification is not necessary for a dust deposit to qualify as loess the terms primary loess and secondary loess have previously been used to described wind deposited loess and redeposited loess respectively but it is more accurate to describe material reworked and redeposited by non aeolian processes as loess derived colluvium or loess derived alluvium during the quaternary loess formed in three main situations 1 mid continental shield areas beyond the limits of ice sheets periglacial loess 2 on the margins of high mountain ranges perimontane loess and 3 on the semi arid margins of some lowland deserts peridesert loess peridesert loess is relatively poorly developed partly due to the limited efficiency of silt generating mechanisms in low land deserts compared with glaciation but also important is the tendency in arid areas for dust to be dispersed over a wide area and to be rapidly reworked after deposition particularly in areas which experience frequent climatic changes the remarkably thick loess of china and central asia results from an unusual combination of conditions which has persisted for much of the last 2 3 million years namely rapid uplift of the tibetan plateau and surrounding mountain ranges high rates of sediment production and supply to adjacent basins a strong northwesterly and westerly wind regime and the existence of effective dust traps downwind of the source regions

an isochron method for cosmogenic nuclide dating of buried soils and sediments we describe an improved method for dating buried paleosols using measurements of the cosmic ray produced radionuclides 10be and 26al in quartz grains and apply it to a sequence of intercalated tills and paleosols in central missouri usa that record plio pleistocene advances of the laurentide ice sheet a buried paleosol implies a period of surface exposure and nuclide accumulation followed by burial and a halt to nuclide production if the paleosol is formed in a sedimentary unit such as till this unit may also have been emplaced with unknown 26al and 10be concentrations inherited from past surface exposure if the inherited nuclide concentrations are the same at all depths in the soil as is true for well mixed sediments such as till then the 26al and 10be concentrations at different depths in the paleosol will show a linear relationship the slope of this line depends on the duration of burial of the paleosol but not on the inherited nuclide concentrations or on the sample depths thus one can date strata overlying buried paleosols by measuring 26al and 10be at multiple depths in the paleosol and calculating the burial age of the paleosol from the resulting isochron we focus on applying this approach to till paleosol sequences but the basic idea of forming an 26al 10be burial isochron with a set of samples that share the same burial age but differ in other aspects of their exposure history applies to other stratigraphic settings as well the method yields ages for four tills in missouri that are stratigraphically consistent agree with paleomagnetic age constraints and show that ice advanced into missouri near 1 25 ma near 0 8 ma and twice between ca 0 4 and 0 2 ma

analytic comparison of apparent polar wander paths abstract for geological times older than the age of the sea floor and for which we can justifiably extrapolate specific supercontinent configurations an appeal to palaeomagnetism would seem the most appropriate approach to solve problems of continental distribution in order to improve upon procedures whereby apparent polar wander paths are matched solely by inspection we have developed an iterative technique which allows us to match paths on the surface of a sphere on an objective basis the notion of a minimum combined path length from a comparison of two data sets is used to determine the optimal match up the technique is appropriate to cases where the only dating available for the points on each path is their relative chronology

quantifying denudation rates and sediment storage on the eastern altiplano bolivia using cosmogenic 10be 26al and in situ 14c abstract denudation processes and sediment transfer are investigated in a high elevation low relief environment eastern altiplano bolivia using 10be 26al and in situ 14c analysis in fluvial sediments concentrations of the long lived nuclides 10be and 26al yield consistently low catchment wide denudation rates of 3 29 mm ky 1 integrating over 21 194 ky which reflect the low geomorphic gradients and the discontinuity of fluvial transport along the eastern altiplano margin no significant correlation is recorded between denudation rates of individual catchments and morphological basin parameters slope area elevation this is attributed to the overall little variability in morphology the agreement between the denudation rates and published modern sediment discharge data suggests steady landscape evolution of the eastern altiplano from the latest pleistocene until today while 10be and 26al provide long term estimates on sediment production in situ cosmogenic 14c is used to trace short term sediment storage in situ 14c concentrations are comparatively low indicating that 14c decayed during alluvial storage over at least the past 11 20 ky we assume storage at shallow depth 2 m and consider the influence of soil mantled hillslopes on the in situ 14c concentration our results illustrate the importance of sediment storage even over short distances and demonstrate the potential of in situ 14c to study sediment routing and transfer times within drainage systems however this study also demonstrates that the long lived 10be and 26al nuclides can provide adequate estimates on long term denudation rates even if sediment transport is not fast but interrupted by several thousands of years of storage

ratio correlations and major element mobility in altered basalts and komatiites attempts have been made in the recent literature to constrain major element mobility in altered basalts and komatiites using the molecular proportion ratio mpr diagrams developed by t h pearce 1970 the method involves ratioing the molecular proportions of two oxides against a third oxide and presenting the results as an oxide oxide ratio plot the transformation appears to clean up scatter plots with low correlations and produces highly correlated ratio plots these plots have been taken to be indicative of igneous fractionation trends and thus of minimal bulk chemical change during alteration there are however inherent statistical problems in ratio correlation which have received much attention in the mathematical literature this paper explains the implications of enhanced correlations for work on altered rocks using ratio plots and suggests that extreme caution must be exercised in the interpretation of mpr diagrams

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

holocene paleohydrology and glacial history of the central andes using multiproxy lake sediment studies here we document at century to millennial scale the regional changes of precipitation evaporation from the late pleistocene to present with multiproxy methods on a north south transect of lake sites across the eastern cordillera of the central andes the transect of study sites covers the area from v14 s to 20 s and includes core studies from seven lakes and modern calibration water samples from twenty three watersheds analyzed to constrain the down core interpretations of stable isotopes and diatoms we selected lakes in different hydrologic settings spanning a range of sensitivity to changes in the moisture balance these include 1 lakes directly receiving glacial meltwater 2 overflowing lakes in glaciated watersheds 3 overflowing lakes in watersheds without active glaciers and 4 lakes that become closed basins during the dry season the results of our current work on multiple lakes in the bolivian andes show that while the overall pattern of holocene environmental change is consistent within the region conditions were not always stable over centennial to over millennial timescales and considerable decadal to centuryscale climate variability is evident abbott et al quat res 47 1997 70 80 quat res 47 1997 169 180 quat sci rev 19 2000 1801 1820 polissar master s thesis university of massachusetts 1999 comparison of the paleoclimate record from one well studied site lago taypi chaka kkota ltck with others within the region illustrates a consistent overall pattern of aridity from the late glacial through the middle holocene previous work noted a difference between the timing of water level rise in lake titicaca v5 0 3 5 ka b p abbott et al quat res 47 1997 169 180 cross et al holocene 10 2000 21 32 rowe et al clim change 52 2002 175 199 and the onset of wetter conditions at 2 3 ka b p in ltck a lake that drains into the southern end of lake titicaca abbott et al quat res 47 1997 70 80 sedimentary and oxygen isotope evidence from paco cocha 13 54ps located in the northern reaches of the expansive 57 000 km 2 titicaca watershed which spans v14 s to 17 s indicates that

rise of the andes the surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening leading to their gradual rise recent studies of the andes indicate that their elevation remained relatively stable for long periods tens of millions of years separated by rapid 1 to 4 million years changes of 1 5 kilometers or more periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable dense lower lithosphere after long term thickening of the crust and lithospheric mantle

what does a mean mean the temporal evolution of detrital cosmogenic denudation rates in a transient landscape in equilibrium landscapes 10 be concentrations within detrital quartz grains are expected to quantitatively refl ect basin wide denudation rates in transient landscapes though detrital quartz is derived from both the incising adjusting lowland and the unadjusted relict upland the integrated 10 be concentrations still provide a denudation rate averaged across the two domains because fi eld samples can provide only a snapshot of the current upstream averaged erosion rate we employ a numerical landscape evolution model to explore how 10 be derived denudation rates vary over time and space during transient adjustment model results suggest that the longitudinal pattern of mean denudation rates is generated by the river s progressive dilution of low volume high concentration detritus from relict uplands by the integration of high volume low concentration detritus from adjusting lowlands the proportion of these materials in any detrital sample depends on what fraction of the upstream area remains unadjusted because the boundary of the adjusting part of the landscape changes over time the longitudinal trend in cosmogenic nuclide derived erosion rates changes over time these insights are then used to guide our interpretation of geomorphic and longitudinal cosmogenic nuclide data from the south fork eel river sfer in the california coast range united states the northward propagating crustal thickening and rock uplift associated with the passage of the mendocino triple junction generates a mobile wave of uplift that progressively sweeps longitudinally down the sfer the consequences of this forcing can be both replicated in the model environment and observed in the fi eld the sfer contains transient landforms including knickpoints and river terraces along mainstem and tributary channels that defi ne a clear boundary between an incised adjusting lowland and an unadjusted relict upland we report nine nested basin wide denudation rates in the mainstem of the sfer using terrestrial cosmogenic 10 be in river borne sediment we fi nd that denudation rates increase in the downstream direction from 0 2 mm yr in the upper catchment to 0 5 mm yr at the outlet using comparisons to the modeled landscape we show that this pattern of denudation rates paired with the distribution of relict topography throughout the watershed refl ect the immaturity of the landscape s transient adjustment later in this modeled transient the predicted erosion rates decrease downstream before they become uniform this interpretation of our data has potentially far reaching implications for quantifying the uplift history and response time of transient landscapes using cosmogenic nuclides

identifying minerals controlling the chemical evolution of igneous rocks beyond pearce element ratio diagrams abstract a new method for identifying mineral controls on the chemical evolution of magmas has statistical power and interpretative reliability exceeding those of the conserved denominator method conserved denominator or pearce element ratio analysis compares two chemical variables that have each been divided by the same third chemical variable the common divisor introduces induced correlations that tend to reduce statistical power the method tends to assert the presence of mineral controls where none exist no available method can remove or even quantify the effects of these induced correlations by recasting chemical data into a new form standard regression analysis serves to evaluate the same geochemical hypotheses evaluated by common denominator analysis this new approach devolves from simple geometric relationships among compositional variables is consistent with a linear mixing paradigm and does not introduce induced correlations furthermore tests for co control by minerals with common cations e g olivine and orthopyroxene are easily formulated with this new method case studies involving olivine and olivine plagioclase control as well as synthetic data studies illustrate and verify this new method

links between topography erosion rheological heterogeneity and deformation in contractional settings insights from the central andes abstract orogenic structure in the central andes 15 s 34 s systematically varies with mean annual precipitation suggesting that erosional processes may be coupled to tectonic processes we explore the range of possible interactions between deformation erosional processes changing geodynamic conditions and preexisting geologic structures to assess the relative importance of each our review and synthesis indicates that the pre orogenic geologic history and changing geodynamic conditions leave a lasting imprint on the modern structure of the mountain belt in contrast changes in precipitation that result from regional scale atmospheric and oceanic circulation interact with local topography to control the climatic zoning across the central andes changes in erosion which likely correspond to changes in climate rock uplift rate and the exposure of different lithologies may in turn affect the tectonic development of the region at the scale of each morphotectonic province in this view geologic and geodynamic factors set the stage for the nature and strength of coupling between erosional processes tectonic deformation and mountain belt structure in the different parts of the central andes

age erosion constraints on an early pleistocene paleosol in yukon canada with profiles of 10be and 26al evidence for a significant loess cover effect on cosmogenic nuclide production rates abstract wounded moose type paleosols developed on remnant deposits of late pliocene to early pleistocene pre reid cordilleran ice sheet cis glaciations in central yukon canada it is an important regional soil geomorphic marker at the boundary between early cis advances and the non glaciated regions of yukon and alaska yet at present its age is poorly constrained between the reid 0 2 ma and earliest 2 84 ma cis advances here we apply depth profiles of in situ produced cosmogenic 26al and 10be to obtain both a minimum exposure age 1 12 0 44 0 36 ma 2 and maximum erosion rate 1 1 0 9 0 5 m myr 1 for the wounded moose paleosol our results show that this soil formed under exceptionally stable conditions max erosion rate similar to polar bedrock erosion rates and that it pre dates the emergence of the 100 ka eccentricity climate cycle contrasting our results from single and joint nuclide depth profile models reveals a significant discrepancy between calculated and effective 10be and 26al production rates 40 65 of expected values we interpret this discrepancy as the result of intermittent loess cover with a time averaged depth between 60 and 110 cm which significantly reduced apparent exposure ages obtained from the single nuclide model the observation of such a significant loess cover effect on cosmogenic nuclide production has implications for exposure dating in glacial and periglacial environments a multi nuclide sampling strategy is required to quantify this effect

sediment production and delivery in the amazon river basin quantified by in situ produced cosmogenic nuclides and recent river loads we use cosmogenic nuclide derived denudation rates from in situ produced 10 be in river sediment to determine sediment production rates for the central amazon river and its major tributaries recent developments have shown that this method allows calculating denudation rates in large depositional basins despite intermediate sediment storage with the result that fluxes of the sediment producing hinterland can now be linked to those discharged at the basins outlet in rivers of the central amazonian plain sediment of finer grain sizes 125 500 m yields a weighted cosmogenic nuclide derived denudation rate of 0 24 0 02 mm yr that is comparable to the integrated rate of all main andean draining rivers 0 37 0 06 mm yr which are the beni napo mamore ucayali and maranon rivers coarser grained sediment 500 m of central amazonian rivers is indicative of a source from the tectonically stable cratonic headwaters of the guyana and brazilian shields for which the denudation rate is 0 01 0 02 mm yr respective sediment loads can be calculated by converting these cosmogenic nuclide derived rates using their sediment producing areas for the amazon river at obidos a sediment production rate of 610 mt yr results non andean source areas contribute only 45 mt yr a comparison with published modern sediment fluxes shows similarities within a factor of 2 with an average gauging derived sediment load of 1000 mt yr at obidos for example we attribute this similar trend in cosmogenic versus modern sediment loads first to the absence of long term deposition within the basin and second to the buffering capability of the large amazon floodplain the buffering capability dampens short term high amplitude fluctuations climatic variability in source areas and anthropogenic soil erosion by the time the denudation rate signal of the hinterland is transmitted to the outlet of the basin

26al and10be dating of late pleistocene and holocene fill terraces a record of fluvial deposition and incision colorado front range cosmogenic 26al 10be and 14c dating of fluvial fill terraces in steep canyons of the colorado front range provides a temporal framework for analysing episodic aggradation and incision results from boulder canyon show that terrace heights above the modern channel grade can be divided into 1 bull lake 100 ka 20 15 m above grade 2 pinedale 32 10 ka 15 4 m above grade and 3 holocene age 15 m above grade record bull lake deposition well preserved terraces of pinedale age suggest that the range of terrace height above grade reflects short term fluctuations in the river profile during periods of rapidly changing stream load and power net river incision apparently occurred during transitions to interglacial periods soil development and stratigraphic position along with limited cosmogenic and 14c dating suggest that 130 ka terraces in boulder canyon correlate with the louviers alluvium and that 32 to 10 ka fills in the canyon correlate with the broadway alluvium on the adjacent high plains late pleistocene incision rates 0 15 m ka 1 along boulder canyon exceed pre late pleistocene incision rates and are higher than middle to late pleistocene incision rates 0 04 m ka 1 on the high plains this study provides an example of how modern geochronologic techniques allow us to understand better rivers that drain glaciated catchments copyright 2002 john wiley sons ltd

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

recent glacial history and paleoclimate of the peruvian bolivian andes abstract radiocarbon dates of glacial deposits in the peruvian bolivian andes are used to place the observed phases of glaciation in a common chronological framework the climatic conditions that may determine modern snowline are examined in order to understand the types of climatic forcing to which glaciers may have been sensitive in the past also considered are independent proxies of climatic change including pollen sequences lake level history and ice core data late pleistocene glaciation in central peru between 24 ka and 12 ka bp appears to have occurred during colder and perhaps drier conditions in bolivia a glacial maximum is dated to phases of late holocene glaciation in peru that correspond in part to the little ice age in the northern hemisphere appear to have been caused by a combination of colder and wetter conditions an overall warming trend may be the cause for the retreat of many glaciers in the region since the mid 19th century

explicit treatment of inheritance in dating depositional surfaces using in situ 10be and 26al we describe a new strategy for dating depositional landscape surfaces using in situ produced cosmogenic radionuclides crns that removes the complication of nuclide inheritance by clasts prior to depositon two amalgamated samples each consisting of 30 clasts one from the surface and one from a fixed depth in the subsurface constrain this crn inheritance and date the surface the inheritance may be used to estimate minimum exhumation rates and maximum transport times within the geomorphic system we test the technique using sup 10 be and sup 26 al to date the third fr3 of five terraces along the fremont river utah and the third wr3 of 15 along the wind river wyoming whereas effective ages based solely upon the surface samples yield 118 138 ka fr3 and 93 ka wr3 the subsurface samples reveal that inheritance accounts for 22 wr3 to 43 fr3 of the total crn concentration taking this into account yeilds terrace ages of nearly 61 81 ka for fr3 and nearly 67 76 ka for wr3 we explore the dependence of age estimates on the accumulation history of the terrace silt caps 22 refs 3 figs 1 tab

estimating rates of denudation using cosmogenic isotope abundances in sediment we propose as a testable hypothesis a basin scale approach for interpreting the abundance of in situ produced cosmogenic isotopes an approach which considers explicitly both the isotope and sediment flux through a drainage basin unlike most existing models which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on earth s surface our model is designed for interpreting isotope abundance in sediment because sediment is a mixture of materials in favourable cases derived from throughout a drainage basin we suggest that measured isotope abundances may reflect spatially averaged rates of erosion we investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed

orographic barriers high resolution trmm rainfall and relief variations along the eastern andes 1 quantifying the degree to which tectonic and erosive processes shape landscapes is key to understanding the evolution of tectonically active mountain belts here we explore the interplay of these two processes along the humid eastern flank of the south american andes we use high resolution tropical rainfall measurement mission trmm and srtm data to characterize elevation relief and hillslope angle of peak rainfall at orographic barriers over a distance of more than 3500 km along the eastern flanks we find that peak rainfall 3 5 m yr occurs at a mean elevation of 1 3 0 17 km a mean relief of 0 95 0 08 km and at moderate mean hillslope angles of 18 3 1 7 we suggest that topographic relief is the best first order rainfall predictor and we demonstrate how relief changes along strike south of the andean orocline alter rainfall distribution changes in climatic sedimentary and tectonic processes prevent the formation of high relief amounts at the mountain front and these areas are not characterized by pronounced rainfall peak

regional synthesis of last glacial maximum snowlines in the tropical andes south america the modern glaciers of the tropical andes are a small remnant of the ice that occupied the mountain chain during past glacial periods estimates of local last glacial maximum lgm snowline depression range from low e g 200 300 m in the junin region peru through intermediate 600 m at laguna kollpa kkota in bolivia to high e g 1100 1350 m in the cordillera oriental peru although a considerable body of work on paleosnowlines exists for the tropical andes absolute dating is lacking for most sites moraines that have been reliably dated to 21 cal kyr bp have been identified at few locations in the tropical andes more commonly but still rarely moraines can be bracketed between about 10 14 c kyr 11 5 cal kyr bp and 30 14 c kyr bp typically only minimum limiting ages for glacial retreat are available cosmogenic dating of erratics on moraines may be able to provide absolute dating with sufficient accuracy to identify deposits of the local lgm ongoing work using cosmogenic 10 be and 26 al in peru and bolivia suggests that the local lgm may have occurred prior to 21 cal kyr bp r 2005 elsevier ltd and inqua all rights reserved

hydrologic variation during the last 170 000 years in the southern hemisphere tropics of south america despite the hypothesized importance of the tropics in the global climate system few tropical paleoclimatic records extend to periods earlier than the last glacial maximum lgm about 20 000 years before present we present a well dated 170 000 year time series of hydrologic variation from the southern hemisphere tropics of south america that extends from modern times through most of the penultimate glacial period alternating mud and salt units in a core from salar de uyuni bolivia reflect alternations between wet and dry periods the most striking feature of the sequence is that the duration of paleolakes increased in the late quaternary this change may reflect increased precipitation geomorphic or tectonic processes that affected basin hydrology or some combination of both the dominance of salt between 170 000 and 140 000 yr ago indicates that much of the penultimate glacial period was dry in contrast to wet conditions in the lgm our analyses also suggest that the relative influence of insolation forcing on regional moisture budgets may have been stronger during the past 50 000 years than in earlier times d 2003 university of washington all rights reserved

using cosmogenic nuclides to contrast rates of erosion and sediment yield in a semi arid arroyo dominated landscape rio puerco basin new mexico analysis of in situ produced 10 be and 26 al in 52 fluvial sediment samples shows that millennialscale rates of erosion vary widely 7 to 366 m ma 1 through the lithologically and topographically complex rio puerco basin of northern new mexico using isotopic analysis of both headwater and downstream samples we determined that the semi arid rio puerco basin is eroding on average about 100 m ma 1 this rapid rate of erosion is consistent with estimates made using other techniques and is likely to result from a combination of easily eroded lithologies sparse vegetation and monsoon dominated rainfall data from 331 stream water samples collected by the us geological survey between 1960 and 1995 are consistent with basin wide average chemical denudation rates of only about 1 4 m ma 1 thus the erosion rates we calculate may be considered rates of sediment generation because physical weathering accounts for almost 99 per cent of mass loss the isotopic data reveal that sediment is generally well mixed downstream with the areaweighted average sediment generation rate for 16 headwater samples 234 ton km 2 a 1 for

neotectonic effects on sinuosity and channel migration belle fourche river western south dakota short term instability in the behaviour of a small meandering alluvial channel is identified from the relation between sinuosity and either floodplain slope or channel slope within 17 reaches along an 81 kilometre section of the belle fourche river in western south dakota in reaches 1 to 4 and 11 to 17 the channel is relatively stable and sinuosity varies inversely with channel slope in reaches 5 to 10 sinuosity is positively related to floodplain slope sinuosity increases markedly in reaches 5 6 and 7 which are immediately downstream from a discontinuity in the long profile of the floodplain in association with an increase in floodplain slope immediately upstream from the discontinuity bankfull channel depth and sinuosity decrease and the area of the floodplain reworked by meander migration between 1939 and 1981 increases in association with a decrease in floodplain slope channel behaviour in reaches 5 to 10 is best explained as a consequence of neotectonic activity as indicated by changes in elevation recorded along geodetic survey lines that cross lineaments that may delimit the eastern boundary of the black hills uplift sinuosity acts as a barometer of the effects of neotectonic activity on alluvial channels initial indications of channel and floodplain instability due to neotectonic activity may be derived from evidence of anomalously active channel migration as documented from photographic or topographic sources

constraining processes of landscape change with combined in situ cosmogenic 14c 10be analysis abstract reconstructing quaternary landscape evolution today frequently builds upon cosmogenic nuclide surface exposure dating however the study of complex surface exposure chronologies on the 102 104 years timescale remains challenging with the commonly used long lived radionuclides 10be 26al 36cl in glacial settings key points are the inheritance of nuclides accumulated in a rock surface during a previous exposure episode and partial shielding of a rock surface after the main deglaciation event e g during phases of glacier readvance combining the short lived in situ cosmogenic 14c isotope with 10be dating provides a valuable approach to resolve and quantify complex exposure histories and burial episodes within lateglacial and holocene timescales the first studies applying the in situ 14c 10be pair have demonstrated the great benefit from in situ 14c analysis for unravelling complex glacier chronologies in various glacial environments worldwide moreover emerging research on in situ 14c in sedimentary systems highlights the capacity of combined in situ 14c 10be analysis to quantify sediment transfer times in fluvial catchments or to constrain changes in surface erosion rates nevertheless further methodological advances are needed to obtain truly routine and widely available in situ 14c analysis future development in analytical techniques has to focus on improving the analytical reproducibility reducing the background level and determining more accurate muonic production rates these improvements should allow extending the field of applications for combined in situ 14c 10be analysis in earth surface sciences and open up a number of promising applications for dating young sedimentary deposits and the quantification of recent changes in surface erosion dynamics

the identification and assessment of spurious trends in pearce type ratio variation diagrams a discussion of some statistical arguments spurious trends in pearce type ratio variation diagrams are constrained to go through the origin and the average composition of the data set a valid trend has a significant nonzero y intercept and a meaningful slope the two types of trends are thus easily distinguished numerical simulation is a useful perhaps even necessary method of checking the effects of random variations calculations of probability suggest that the odds against purely random data mimicking an olivine trend in komatiites are great perhaps as much as 100 00 0000 to 1 that such original igneous trends are descernible through the veil of metamorphic overprinting is a powerful demonstration of the usefulness of this technique

geoid pangea and convection abstract we show that the present geoid has a simple low order configuration with an axis of symmetry in the equatorial plane we show further that it is a tennis ball pattern with an equatorial high belt and a polar low one which is clearly controlled by the rotation of the earth finally we show that the outline of pangea between at least 200 ma and 125 ma ago lay along a great circle passing through the paleo poles of rotation thus it also had an axis of symmetry in the equatorial plane this hemispheric super continent configuration ended in middle cretaceous time during a major geologic catastrophe which was accompanied by high rates of spreading hotspot outbreaks and high sea level stands we interpret this evidence in terms of separate steady state lower mantle convection responsible for the present geoid weakly coupled to the upper mantle one this weak coupling leads to the hemispheric continent configuration which ends when excessive heating of the upper mantle due to the insulating continental cap leads to continent dispersal the complete cycle from one supercontinent to the next might be of the order of 400 ma

rates of sediment supply to arroyos from upland erosion determined using in situ produced cosmogenic 10be and 26al using 10be and 26al measured in sediment and bedrock we quantify rates of upland erosion and sediment supply to a small basin in northwestern new mexico this and many other similar basins in the southwestern united states have been affected by cycles of arroyo incision and backfilling several times in the past few millennia the sediment generation 275 65 g m 2 yr 1 and bedrock equivalent lowering rates 102 24 m myr 1 we determine are sufficient to support at least three arroyo cycles in the past 3 000 years consistent with rates calculated from a physical sediment budget within the basin and regional rates determined using other techniques nuclide concentrations measured in different sediment sources and reservoirs suggest that the arroyo is a good spatial and temporal integrator of sediment and associated nuclide concentrations from throughout the basin that the basin is in steady state and that nuclide concentration is independent of sediment grain size differences between nuclide concentrations measured in sediment sources and reservoirs reflect sediment residence times and indicate that subcolluvial bedrock weathering on hillslopes supplies more sediment to the basin than erosion of exposed bedrock

radiocarbon chronology of the mammoth site at niederweningen switzerland results from dating bones teeth wood and peat abstract the peat section at niederweningen is a site of spectacular excavations of numerous bones of mammoth and other animals that was first recognized in the late 19th century in 1987 1990 new excavations provided more peat material which was radiocarbon dated to 33 000 14 c years bp and 42 000 14 c years bp the first ams radiocarbon ages of mammoth bones from 1890 1891 excavations placed the age between 33 000 14 c years bp and 35 000 14 c years bp in this study new results were obtained on bones teeth wood and peat from the layer that buried the mammoths the ages confirm that the mammoths as well as the peat section in which they were found are older than 40 000 14 c years bp in order to explain the discrepancy between the previous chronology and the new results two preparation techniques were applied for the dating of bones an organic fraction called collagen used to obtain the first radiocarbon ages was dated as well as gelatine prepared using the modified longin method the differences of up to 7000 years between ages obtained on collagen and gelatine fractions show contamination of the collagen fraction with modern carbon the oldest age of 45 870 1080 14 c years bp was obtained on base cleaned gelatine from mammoth bone this age is close to an age of 45 430 1020 14 c years bp obtained for the peat layer that buried the mammoths

late quaternary glacier advances and palaeolake highstands in the bolivian altiplano abstract recent study of late quaternary moraines and pluvio lacustrine features around the southern altiplano of bolivia suggests they developed more or less synchronously during the last glacial cycle the stratigraphy of lacustrine and deltaic sediments can be linked to outwash and moraines on glaciated massifs e g cerro condor iquina cerro azanaques and cerro tunupa which border the poopo uyuni and coipasa salt basins uranium series and radiometric dating of lake carbonates by previous workers suggests that major palaeolakes developed during the intervals 72 68 ka bp 44 34 ka bp and 15 4 9 5 ka bp radiocarbon dating of peat associated with glacifluvial and glacial sediments indicates that the most extensive glacier advance of the later part of the last glacial cycle culminated after 13 3 ka bp and that a smaller readvance may have occurred between 12 and 10 ka bp as the highest palaeolake stand the tauca phase has been dated to ca 13 8 ka bp it is concluded that increased effective moisture and cool climatic conditions during the interval ca 14 13 ka bp forced the simultaneous expansion of glaciers and lakes in and around the southern altiplano a palaeolake may have been present on the southern altiplano for much of the interval ca 15 4 9 5 ka bp its surface reaching different levels at different times

secular variation of archaeomagnetic direction in the american southwest a d 750 1425 oriented archaeomagnetic samples were collected from 158 in situ features at 33 archaeological sites in the southwestern united states seventy three independently dated features were used for analysis of secular variation a moving window technique with outlier rejection was developed to compute a smoothed secular variation curve this technique incorporates weighted fisher statistics to account for imprecision in both the age of remanence acquisition and the direction of magnetization for each feature a mean direction and measure of dispersion is generated for each window of time the secular variation record covers the period a d 750 1425 during which time the virtual geomagnetic pole vgp moves in a generally counterclockwise fashion from 86 n 83 e at a d 750 to the lowest latitude position of 74 n 192 e at a d 1075 and then to 85 n 236 e at a d 1425 the median rate of vgp movement is 0 036 yr 1 similar to present day rates the southwestern record shows close correspondence to other north american archaeomagnetic limnomagnetic and speleomagnetic records the archaeomagnetic record can serve as the master curve for the derivation of archaeomagnetic dates