chemical and physical denudation in the amazon river basin we present major and trace element data on the suspended and dissolved phases of the amazon river and its main tributaries the sr isotopic composition of the dissolved load is also reported special attention is paid to the abundances of ree and to their fractionation between the dissolved and suspended phase the rivers of the amazon basin are among the richest in dissolved ree and are similar to the rivers of the congo system however a greater range of fractionation between lree and hree is reported here at a global scale the rivers have intermediate patterns between those of the congo system and those of high ph rivers such as the indus and mississippi rivers only few elements rb u ba k na sr and ca are mobilized by silicate weathering these elements are strongly depleted in the suspended phase with respect to upper continental crust in the dissolved load these elements are controlled by atmospheric inputs and the weathering of the main lithologies we propose a model based on mass budget equations that allow the proportions derived from the different sources to be calculated as a consequence silicate carbonate and evaporite weathering rates can be estimated as well as the consumption of co2 by weathering of each of these lithologies physical weathering rates can be estimated by two complementary approaches on the one hand the multi year average of suspended sediments yields can be used to estimate physical denudation on the other hand we have developed a steady state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers a mean crustal composition is assumed in this model for the rock sources of the drainage basins comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers but a strong discrepancy for the rivers derived from the andes andean rivers solimoes madeira and amazon have observed sediment yields much greater than those predicted according to the steady state model of chemical and physical weathering two interpretations can account for this inconsistency the first is that these rivers are not in steady state and hence that the soils are being destroyed the second requires that the local continental crust is different from the average continental crust of taylor and mclennan and contains a large proportion of sedimentary rocks using the measured sediment yields and assuming a steady state we can estimate the amount of sediment recycling for each drainage basin for the amazon at santarem we find that at least 25 of the mass of the upper continental crust of the amazon drainage basin is constituted of recycled material

neogene and quaternary history of vegetation climate and plant diversity in amazonia the neotropical amazonian and andean plant diversity developed mainly during the tertiary in amazonia miocene oral diversity seems considerably higher than today during the neogene tropical taxa entered newly created montane area and montane taxa entered the tropical lowlands the general decrease of temperature during the upper neogene and especially during the quaternary glacial periods may have caused considerable extinctions in the lowlands representation of pollen of apparently principally montane taxa podocarpus hedyosmum in miocene pliocene and quaternary sediments of amazonia and surroundings is still di cult to interpret in terms of temperature decrease at low elevation changes in precipitation may have profound impact on the composition of vegetation communities ilex and melastomataceae increase signi cantly in many glacial pollen records increase of weinmannia in amazonian pollen records seems the best indicator of downward migration of montane vegetation belts a temperature lowering at sea level of 4 5 13c during the last glacial maximum lgm seems reasonable it may have caused a downslope migration of some 700 m of the lower montane vegetation belt lower montane arboreal species may have been able to grow in higher elevation areas 500 m of amazonia increasing background pollen values of montane taxa in the area di erence between a cool and wet middle pleniglacial 60 28 ka bp and a cold and dry upper pleniglacial 28 14 ka bp thus including the lgm is evident in andean and amazonian records statements about environmental conditions of the ice age amazon should be speci ed chronologically the middle pleniglacial is a time of accumulation of uvial sediments the upper pleniglacial is a time of incision of the rivers in their sediments sedimentation started again in the lateglacial since ca 13 ka bp and the holocene when lake levels rose again based on simpli ed considerations of precipitation changes and evaporation we estimate that lgm rainfall may have been reduced by values of ca 45 10 amazonian and cordilleran lakes dried up dry rain forest was locally replaced by savanna savanna forest or cerrado type vegetation dry rain forest savanna forest and pure savanna was locally replaced by extensive semi desert dune formations lower rio branco area in present day central amazonia the present day centers of higher rainfall 2500 mm surrounded by areas of lower rainfall are refuge areas of the very wet rain forest and of the very high plant diversity 300 plant species per 0 1 ha and they should have been that equally or more during the dry climate intervals plant diversity of drier forests is in the order of 100 150 species per 0 1 ha both extinction and speciation in isolation under precipitation and temperature stress may have taken place in these refugia 2000 elsevier science ltd all rights reserved

time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

transient sediment supply in a high altitude alpine environment evidenced through a 10be budget of the etages catchment french western alps although beryllium 10 10be concentrations in stream sediments provide useful synoptic views of catchment wide erosion rates little is known on the relative contributions of different sediment supply mechanisms to the acquisition of their initial signature in the headwaters here we address this issue by conducting a 10be budget of detrital materials that characterize the morphogenetic domains representative of high altitude environments of the european alps we focus on the etages catchment located in the ecrins pelvoux massif southeast france and illustrate how in situ 10be concentrations can be used for tracing the origin of the sand fraction from the bedload in the trunk stream the landscape of the etages catchment is characterized by a geomorphic transient state high topographic gradients and a large variety of modern geomorphic domains ranging from glacial environments to scarcely vegetated alluvial plains beryllium 10 concentrations measured in the etages catchment vary from similar to 1 x 104 to 4 5 x 105 atoms per gram quartz while displaying consistent 10be signatures within each representative morphogenetic unit we show that the basic requirements for inferring catchment wide denudation from 10be concentration measurements are not satisfied in this small dynamic catchment however the distinct 10be signature observed for the geomorphic domains can be used as a tracer we suggest that a terrestrial cosmogenic nuclide tcn budget approach provides a valuable tool for the tracing of material origin in basins where the let nature do the averaging principles may be violated

transience and persistence in the depositional record of continental margins 1 continental shelves and coastal plains are large persistent depositional landforms which are stationary nonmigrating at their proximal ends and characterized by relatively steady long term growth in detail however their surface form and stratigraphic record is built of transient freely migrating landscape elements we derive the timescales of crossover from transient to persistent topographic forms using empirical scaling relations for mean sediment accumulation as a function of averaging time based upon tens of thousands of empirical measurements a stochastic noisy diffusion model with drift predicts all the gross features of the empirical data it satisfies first order goals of describing both the surface morphology and stratigraphic completeness of depositional systems the model crossover from noise dominated to drift dominated behavior corresponds to the empirical crossover from transport dominated autogenic transient behavior to accommodation dominated subsidence persistent behavior which begins at timescales of 102 103 years and is complete by scales of 104 105 years because the same long term scaling behavior emerges for off shelf environments it is not entirely explicable by steady subsidence fluctuations in sediment supply and routing probably have significant influence at short term transient scales the exponents of the scaling relations vary with environment particularly the prevalence of channeled sediment transport at very small scales modeling sediment transport as a diffusive process is inappropriate our results indicate that some of the timescales of interest for climate interpretation may fall within the transitional interval where neither accommodation nor transport processes are negligible and deconvolution is most challenging

landscape response to pleistocene holocene precipitation change in the western cordillera peru 10be concentrations in modern sediments and terrace fills 1 the landscape response to climate change is frequently investigated with models because natural experiments on geologic timescales are rare in quebrada veladera in the western andes mountains the formation of alluvial terraces during periods of high precipitation presents opportunities for such an experiment we compare drainage average erosion rates during pleistocene terrace deposition with holocene rates using cosmogenic 10be samples for seven pairs of quartz sand taken from the trunk and tributaries of quebrada veladera and adjacent terraces each pair consists of sediment collected from the modern channel and excavated from an adjacent fill terrace the terrace fill was deposited at 16 ka and preserved an isotopic record of paleoerosion rates in the late pleistocene modern sands yield 10be concentrations between 1 68 105 and 2 28 105 atoms g corresponding to holocene erosion rates between 43 3 and 58 4 mm kyr the 10be concentrations in terrace sands range from 9 46 104 to 3 73 105 atoms g corresponding to paleoerosion rates from 27 2 to 103 8 mm kyr smaller upstream tributaries show a substantial decline in erosion rate following the transition from a wet to dry climate but larger drainage areas show no change we interpret this trend to indicate that the wetter climate drove landscape dissection which ceased with the return to dry conditions as channel heads propagated upslope erosion accelerated in low order drainages before higher order ones this contrast disappeared when the drainage network ceased to expand at that point erosion rates became spatially uniform consistent with the uniformity of modern hillslope gradients

modern sands of south america composition provenance and global significance standard petrographic methods were used to study 604 modern sands from south america of which 351 came from rivers and 253 from beaches in spite of the wide geomorphic contrasts these sands belong to only three great families 1 an immature andean family of lithic arenites rich in volcanic and metamorphic grains that covers about 30 of south america 2 a cratonic association rich in quartz that covers about 62 of the continent and 3 a transitional molasse association which occurs between these two and covers only about 8 of the continent the grand arithmetic average q f rf ratio for the entire continent is 60 11 29 and the area weighted carbonate free average qa fa rfa is 68 10 22 a superior continental estimate and the first ever for an entire continent

dating plio pleistocene glacial sediments using the cosmic ray produced radionuclides 10be and 26al we use the cosmic ray produced radionuclides 26al and 10be to date plio pleistocene glacial sediment sequences these two nuclides are produced in quartz at a fixed ratio but have different decay constants if a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic ray flux the 26al 10be ratio is related to the duration of burial we first attempted to date pre wisconsinan tills by measuring 26al and 10be in fluvial sediments beneath them and applying the method of burial dating which previous authors have used to date river sediment carried into caves this method however requires simplifying assumptions about the 26al and 10be concentrations in the sediment at the time of burial we show that these assumptions are not valid for river sediment in glaciated regions 26al and 10be analyses of such sediment do not provide accurate ages for these tills although they do yield limiting ages in some cases we overcome this difficulty by instead measuring 26al and 10be in quartz from paleosols that are buried by tills we use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried as well as the duration of burial this technique provides a widely applicable improvement on other means of dating plio pleistocene terrestrial glacial sediments as well as a framework for applying cosmogenic nuclide dating techniques in complicated stratigraphic settings we apply it to pre wisconsinan glacial sediment sequences in southwest minnesota and eastern south dakota pre wisconsinan tills underlying the minnesota river valley were deposited 0 5 to 1 5 ma and tills beneath the prairie coteau in eastern south dakota and adjacent minnesota were deposited 1 to 2 ma

large scale erosion rates from in situ produced cosmogenic nuclides in european river sediments abstract we have calculated long term erosion rates of 20 100 mm kyr from quartz contained 10be in the bedload of middle european rivers for catchments ranging from 102 to 105 km2 these rates average over 10 40 kyr and agree broadly with rock uplift incision and exhumation rates historic soil erosion rates and erosion rates calculated from the measured sediment loads of the same rivers moreover our new erosion rate estimates correlate well with lithology and relief however in the regen neckar loire and meuse catchments cosmogenic nuclide derived erosion rates are consistently 1 5 4 times greater than the equivalent rates derived from measured river loads this may be due to the systematic under representation of high magnitude low frequency transport events in the gauging records which cover less than a century alternatively the discrepancy may derive from spatially non uniform erosion and preferential tapping of deeper sections of the irradiation profile a third explanation relates the high cosmogenic nuclide derived erosion rates to inheritance of an elevated pleistocene erosion signal uncertainties associated with the cosmogenic nuclide derived erosion rate estimates are not greater than the potential errors in conventional estimates therefore the cosmogenic nuclide approach is an effective tool for rapid catchment wide assessment of time integrated rates of bedrock weathering and erosion and we anticipate its fruitful application to the quaternary sedimentary record

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

the erosion response to quaternary climate change quantified using uranium isotopes and in situ produced cosmogenic nuclides abstract studying how catchment erosion has responded to past climate change can help us better understand not only how landscape evolution operates but also predict the consequences of future climate change on soil resource availability recent years have seen the development of tools that allow a quantitative assessment of past changes in catchment erosion this work reviews the principles of the application of in situ produced cosmogenic nuclides and uranium isotopes to quantifying past erosion rates results highlight the role of periglacial processes and mass wasting in dictating how catchment erosion responds to climatic variability at the 10 kyr scale at the million year scale it is more difficult to untangle the role of climate and tectonics a strong coupling exists at the 10 kyr to 100 kyr scales between climatic cycles and the transfer time of regolith from source to sink this coupling reflects changes in sediment source that are either set by changes in vegetation cover at the catchment scale or by the storage of sediments on continental shelves at a larger scale although further analytical developments are required for these tools to reach their full potential existing works suggest that in the near future they will provide unprecedented quantitative insights on how soil and fluvial systems adapt to external perturbations climatic tectonic and or anthropic

the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment the study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape these measurements are done with in situ produced cosmogenic nuclides e g 10be 26al mostly in quartz from alluvial sediment cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the earth s surface they are produced within a characteristic depth scale of about 1 m which means that the measured concentrations record an integrated denudation history while material passed through this depth interval depending on the denudation rate the resulting integration time scales are 103 to 105 years and one obtains a robust long term estimate of natural denudation that is relatively insensitive to short term changes the last 10 years have seen significant research activity using these methods and an array of fascinating tectono geomorphologic and geochemical insights are emerging amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change a compilation of world wide denudation rates in non glaciated areas that however does not yet include some of the world s most active mountain belts has resulted in the following findings some of which have been unexpected 1 no obvious relationship between precipitation or mean annual temperature and total denudation is apparent 2 topographic relief alone does not result in high rates of denudation 3 denudation rates are high in areas of landscape rejuvenation that is triggered and controlled by tectonic activity faulting escarpment formation and retreat rifting surface uplift 4 rates of weathering using a combination of cosmogenic nuclides and zirconium normalised cation loss balances co vary primarily with physical erosion rates and much less with temperature or precipitation 5 in some areas of high land use short term rates from river load gauging exceed those from cosmogenic nuclides by several orders of magnitude which serves to highlight the severity of geomorphic change caused by human action in the future the control mechanisms over denudation will be determined on all spatial scales ranging from the single soil section to entire river basins the same analysis can be done back through time on well dated terraces lake records and marine sediment cores which is possible with 10be for the past 1 2 my the rates obtained will be used to develop a quantitative understanding of tectonic geomorphologic and geochemical landscape processes which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic climate and anthropogenic forcing

the impact of quaternary sea level changes on the evolution of the amazonian lowland coring and acoustic surveying 3 5 khz in both rivers and lakes in central amazonia provide additional insights into the late quaternary hydrological and sedimentological development of the amazon river and its tributaries erosion and accumulation phases were found to be linked to quaternary sea level changes the low sea level phase during the last glacial maximum caused deep incision of the amazon river and erosion in major tributaries such as the rio negro 1500 km upstream from the amazon mouth a 3 5 khz profiling suggests a lowering of the water level by at least 30 m at manaus during that phase the slope of the amazon valley must have increased resulting in an increase in bed load transport capacity the subsequent sea level rise caused a backwater effect far upstream with silting up of the amazon valley and the tributary inflows former river systems changed into ria lakes the floodplains of the amazon river the varzea were formed approximately 5000 years ago when the sea level approached its present day level copyright 2009 john wiley sons ltd

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

the sediment delivery problem abstract the linking of on site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet and improved knowledge and representation of the associated processes of sediment delivery represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies this paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted the significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced and the need for further work in this field is emphasised

a multiproxy palaeoecological record of holocene lake sediments from the rio tapajos eastern amazonia abstract two sediment cores up to 42 m in length were raised from the wide deep section of the lower tapajos river amazonia referred to as lago tapajos these cores reveal a history of subtle environmental change that began with the formation of lago tapajos as sea level rose about 11 000 years ago the sediments of the lake were deposited fairly quickly at a rate of ca 4 m per millennium and are uniformly fine grained with low organic content the fossil pollen record derived from these sediments reveals that forest surrounded this site throughout the holocene the largest change in the core took place between ca 5500 and 4200 cal years bp and reflects a transition from coarser to finer sediments coincident with the change in sediment is a slight transition in the pollen spectra with an increase in poaceae abundance at the expense of the pioneer tree cecropia cecropia a tentative explanation is offered in which increased human activity possibly spurred by climatic change resulted in the formation of some local grasslands however despite the apparent actions of humans there is no indication of basin wide transformation of landscapes in this record the holocene persistence of forest as the dominant landscape matrix around lago tapajos is supported by low 13 c values and by the constant geochemistry and mineralogy of the lake sediments

10be and 26al production rates deduced from an instantaneous event within the dendro calibration curve the landslide of kofels otz valley austria surface exposure dating requires the knowledge of cosmogenic nuclide production rates when determining time integrated production rates the exposure ages of the calibration samples need to be accurately known the landslide of kofels austria is very well suited for this purpose it is the largest landslide in the crystalline alps of austria dating back to 7800 100 years bc ams 14c dating of buried wood which is well within the 14c dendro calibration curve exposed quartz veins were sampled from the tops of large boulders from the toe of the landslide for analysis of 10be and 26al to calculate sea level high geomagnetic latitude 60 open sky radionuclide production rates corrections were applied for altitude and latitude for shielding by surrounding mountains for sample geometry vegetation and snow cover and for sample thickness the production rates for an exposure age of 10 000 years are 5 75 0 24 10be atoms yr g sio2 and 37 4 1 9 26al atoms yr g sio2 a 26al 10be ratio of 6 52 0 43 can be calculated the influence of the geomagnetic field on these production rates has been estimated using two different geomagnetic field records our production rates should be a good approximation for the use of surface exposure dating between about 5000 and 13 000 years bp

an improved experimental determination of cosmogenic 10be 21ne and 26al 21ne production ratios in quartz abstract the confidence in surface exposure dating and related research such as erosion rate studies or burial dating strongly depends on the accuracy and precision of the currently used production rates of in situ produced cosmogenic nuclides reducing the uncertainties of nuclide production rates by more accurate calibrations with independently dated natural rock surfaces is crucial for further improving the quantification of earth surface processes here we use surface samples from the 760 2 ka old bishop tuff in eastern california to quantify the 10 be 21 ne and 26 al 21 ne production rate ratios in quartz our determination is based on 1 measured nuclide concentrations of cosmogenic 10 be 21 ne and 26 al 2 a conservative estimate for the erosion of the tuff in the volcanic tableland area which we base on our previously published 21 ne concentrations goethals m m niedermann s hetzel r fenton c r 2009 determining the impact of faulting on the rate of erosion in a low relief landscape a case study using in situ produced 21 ne on active normal faults in the bishop tuff california geomorphology 103 401 413 and a conservative estimate for the uncertainty of the 21 ne production rate and 3 the assumption of steady state erosion other assumptions such as the applied scaling procedure the muon contribution to nuclide production or the attenuation lengths of neutrons and muons in rock do not substantially affect the results based on 13 samples the following average production rate ratios and conservative uncertainty estimates are obtained for sea level high latitude open sky and rock surface 0 249 0 009 or 0 232 0 009 for 10 be 21 ne using 10 be half lives of 1 51 and 1 39 ma respectively and 1 80 0 09 for 26 al 21 ne for an 26 al half life of 0 72 ma the 10 be 21 ne and the 26 al 21 ne production ratios are consistent with currently used production rates but the ratios are much more precise than previous determinations the resulting 26 al 10 be production ratio of 7 23 0 45 for a 10 be half life of 1 51 ma or 7 76 0 49 for a 10 be half life of 1 39 ma is high when compared to previously published values we discuss reasons for this difference amongst them the possibility that 26 al analyses in general might be compromised by artefacts affecting the stable al concentration measurements when combined with a 10 be production rate of 5 01 at g 1 a 1 for the 1 51 ma half life or 4 61 at g 1 a 1 for the 1 39 ma half life our production ratios convert to 21 ne and 26 al production rates of 20 1 and 36 2 at g 1 a 1 or 19 9 and 35 7 at g 1 a 1 respectively

decadal changes in the relationship between the tropical pacific and the north pacific 1 it has been recognized that the tropical pacific sea surface temperature sst anomaly associated with the el nino southern oscillation enso is closely tied to large scale atmospheric anomalies in the extra tropical region based on observational data this connection appears to have changed significantly since 1999 this study examines the characteristics of this connection during two sub periods 1980 1998 and 1999 2010 using cyclostationary empirical orthogonal function cseof analysis we found that the atmospheric anomalies induced by the enso alter the sst over the north pacific through surface heat flux change during the earlier period of 1980 1998 during the later period in contrast enso related atmospheric anomalies feature the north pacific oscillation npo pattern during winter which is characterized by a southward shift of the atmospheric center of action from its climatological position the npo related atmospheric anomalies extend to the subtropical pacific this extension potentially links midlatitude and tropical pacific variability through air sea interactions the physical change appears to alter the el nino characteristics into that of the central pacific el nino through the wind sst coupling mechanism this process may be related to the recent change in which the central pacific el nino occurs more frequently than the conventional eastern pacific el nino detailed descriptions of the physical changes between the tropical pacific and the north pacific are presented in this study along with the possible implications of this change

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

short lived u and th isotope distribution in a tropical laterite derived from granite pitinga river basin amazonia brazil application to assessment of weathering rate we have analyzed samples of a 15 m thick profile weathered from the madeira granite located in the pitinga basin river north of manaus in the state of amazonia brazil this profile consists essentially of a yellow red saprolite covered by a soil u and th concentrations are particularly high in the granite 20 and 80 g g respectively normalized element to th concentrations indicate that th is most resistant to chemical weathering except to some extent in the top soil higher concentrations in the saprolite compared to the granite comprise a relative enrichment resulting from a loss of mass the saprolites are initially generated by a descending weathering front which alters the granite to a yellow red saprolite a second front close to the top turns the saprolite into a soil weathering has led to leaching of u the 234u 238u and 230th 238u isotopic ratios are in radioactive disequilibrium numerous nodules are present and apparently started to form at the base of the saprolite these nodules achieve more developed form during their relative ascent until they are reached by the descending top front where they undergo dissolution the th and pb are concentrated in the nodules close to the top front the u being more mobile is strongly leached by the first front and most of the remainder freed by the second engages in a descending flux which supplies the underlying saprolite using the data an attempt is made to model the isotopic distribution in the profile we conclude that the first front has descended at a rate of 5 cm 1000 yt and that the time needed to create the saprolite must have been around 300 000 yr

thermochronometer record of central andean plateau growth bolivia 19 5 s 1 quantifying the timing magnitude and rates of exhumation and deformation across the central andes is a prerequisite for understanding the history of plateau rise we present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire 500 km andean fold thrust belt at 19 5 s in bolivia exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data regional stratigraphy geothermal gradients and mass deficits inferred from a balanced section results suggest the following 1 initial exhumation of the eastern cordillera ec fore thrust and back thrust belts began in the late eocene to early oligocene 27 36 ma and continued in a distributed manner in the late oligocene to early miocene 19 25 ma interandean zone ia exhumation began 19 22 ma followed by a third pulse of exhumation 11 16 ma in the ec back thrust belt and initial cooling in the westernmost subandes sa 8 20 ma finally exhumation propagated eastward across the sa during the late mio pliocene 2 8 ma 2 exhumation magnitudes are spatially variable and range from maximums of 8 km in the ec fore thrust belt to average values of 4 7 km across the ec 2 5 3 km in the altiplano 4 6 km in the ia and 3 km in the sa 3 exhumation rates range from 0 1 to 0 2 mm a in the ec from 0 1 to 0 6 mm a in the ia and from 0 1 to 0 4 mm a to locally 1 4 mm a or more in the eastern sa we synthesize similar constraints with sediments throughout bolivia and characterize plateau development by 1 distributed deformation throughout the altiplano and ec regions from 20 to 40 ma with minor deformation continuing until 10 ma 2 contemporaneous cessation of most ec deformation and exhumation of the ia 20 ma implying establishment of the modern plateau width with significant but unknown crustal thickness and elevation shortly thereafter by 15 20 ma and 3 dominantly eastward propagation of deformation from the ia since 20 ma with minor out of sequence deformation in the central to eastern sa

chemical weathering of fluvial sediments during alluvial storage the macuapanim island point bar solimoes river brazil abstract modification of sediment composition by chemical weathering depends both on the overall intensity of weathering and on its duration at macuapanim island solimboes river amazon main channel brazil a number of alluvial deposits have been exposed to intense tropical weathering for differing lengths of time in the very fine sand fraction the detrital component shows a steady and marked increase in compositional maturity with increasing sediment age immature sands of arkosic to litharenitic composition show a progressive loss of lithic fragments feldspars and accessory minerals with a concomitant enrichment in quartz this process produces first cycle sands whose detrital component is of quartz arenite composition among the salts quartz is enriched over time relative to more nstable phases particularly plagioclase this enrichment is probably largely caused by the destruction of plagioclase through chemical weathering but silt size quartz may also be produced through the breakdown of sedimentary and metasedimentary lithic fragments in the sand fraction in the clay fraction an increase in the relative abundance of smectite accompanies the breakdown of volcanic lithic fragments in the sand fraction and probably reflects the degradation of volcanic glass the clay fractions of the oldest samples examined contain less smectite somewhat more kaolinite and markedly more vermiculite as compared to younger samples but an extensively leached cation poor clay fraction analogous to the quartz dominated silts and sands is never attained the compositional mat rity of the clay fraction is everywhere lower than that of associated silts and sands perhaps because of a continual influx into the clay fraction of immature cation rich clays derived from the physical breakdown of lithic fragments in coarser size fractions this effect is enhanced by the translocation of fines whereby younger less weathered fine material is juxtaposed with older more mature coarse material in the lower levels of the alluvial stratigraphy reworking of floodplains during channel migration results in the reincorporation of older material stored on the floodplains into the sediment load if sediments have been intensely weathered during storage on the floodplain then the result will be a net dilution and replacement of younger relatively immature sands transported by the river by older more mature material from the floodplain the relative proportion of the older sands in the bedload will increase downstream resulting in a progressive downstream increase in the compositional maturity of the bedload of the river

composition and trace element geochemistry of detrital clay and heavy mineral suites of the lowermost amazon river a provenance study abstract compositions of clays and heavy minerals as well as trace element geochemistry of the lowermost amazon sediments reflect source regions and the intense chemical weathering conditions illite and montmorillonite indicate mountainous sources under physical weathering conditions whereas kaolinite is related to the soil composition under humid tropical conditions where chemical weathering predominates the suite of unstable heavy minerals on the other hand reflects some sediment transport from the andes more supply from tributaries draining the basic and metamorphic rocks from the southeastern guiana shield and strongest influence of the upstream amazon and solimoes areas these source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain the brazilian shield contribution via the xingu river seems to be of minor importance although the alter do chao and the barreiras formations which are products of erosion of the shields supply suites of stable heavy minerals that reflect recycling rare earth and trace element data from the clay and heavy mineral fractions provide additional clues to the source areas in the downstream direction superimposed upon provenance of sediments from an undifferentiated arc the andes is provenance from strongly weathered continental crust the precambrian shields as well as sedimentary recycling small scale provenance can be delineated by the most immobile elements heavy rare earth elements in the heavy mineral fraction increase downstream toward estreitos to the para river

river tributaries as sediment sinks processes operating where the tapajos and xingu rivers meet the amazon tidal river the amazon river is the largest fluvial source of freshwater and sediment to the global ocean and has the longest tidally influenced reach in the world two major rivers the tapajos and xingu enter the amazon along its tidal reach however unlike most fluvial confluences these are not one way conduits through which water and sediment flow downstream toward the sea the drowned river valleys rias at the confluences of the tapajos and xingu with the amazon river experience water level fluctuations associated not only with the seasonal rise and fall of the river network but also with semidiurnal tides that propagate as far as 800 km up the amazon river superimposed seasonal and tidal forcing distinct sediment and temperature signatures of amazon and tributary waters and antecedent geomorphology combine to create mainstem tributary confluences that act as sediment traps rather than sources of sediment hydrodynamic measurements are combined with data from sediment cores to determine the distribution of tributary derived and amazon derived sediment within the ria basins characterize the sediment transport mechanisms within the confluence areas and estimate rates of sediment accumulation within both rias the tapajos and xingu ria basins trap the majority of the sediment carried by the tributaries themselves in addition to ca 20 mt yr 1 of sediment sourced from the amazon river these findings have implications for the interpretation of stratigraphy associated with incised valley systems such as those that dominated the transfer of sediment to the oceans during lowstands in sea level this article is protected by copyright all rights reserved

from source to sink preserving the cosmogenic 10be derived denudation rate signal of the bolivian andes in sediment of the beni and mamore foreland basins abstract the denudation rate signal of the bolivian andes as measured by cosmogenic 10 be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years this conclusion is drawn from comparing published andean source area denudation rates with new cosmogenic 10 be data as measured in the floodplains of the large beni and mamore basins for the entire beni basin including the sediment producing andes and the vast flooded plains of the foreland the cosmogenic nuclide derived denudation rate is 0 45 0 06 mm yr while that of the mamore basin is 0 55 0 19 mm yr by comparison the respective andean source areas erode at averaged rates of 0 37 0 06 mm yr upper beni and at 0 56 0 09 mm yr upper mamore we notice a decrease in variability of denudation rate with increasing spatial scale as small scale processes are averaged out sediment mixing within the floodplain damps scatter of nuclide derived denudation rates picked up in the source area on the temporal scale a remarkable agreement between cosmogenic nuclide derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide derived denudation rates capture the long term erosional characteristics of the mountain belt the sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area this finding opens the unique possibility of constraining cosmogenic nuclide derived paleo sediment budgets for these large basins as the long term spatially averaged denudation rate signal of the sediment producing area is preserved in sedimentary archives

channel floodplain geomorphology along the solimoes amazon river brazil across the cratonic landscape of brazil the solimoes amazon river transports to its delta plain 1240 mt of suspended sediment derived from andean erosion and reworks another 3200 mt of floodplain sediments distribution of these sediments has resulted in a variable along stream pattern of geomorphology the upstream reaches are characterized by sediment erosion in the main channel and deposition in floodplain channels that are an order of magnitude smaller in discharge than the main channel sediment deposition in and migration of the floodplain channels erases oxbow lakes of the main channel and yields an intricate scroll bar topography that forms the boundaries of hundreds of long narrow lakes in contrast downstream reaches are characterized by channels restricted by stabilizing long term levee building and floodplain construction dominated by overbank deposition overbank deposition buries the scroll bar topography resulting in a flat floodplain covered by a patchwork of large more equant shallow lakes on the basis of estimated rates of recycling of floodplain sediments the modern floodplain of the brazilian amazon could have been recycled in 5000 yr and is recycled more rapidly in the upstream than the downstream reaches the cratonic interior is interrupted by structural arches that bound intracratonic basins four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel floodplain geomorphology at this spatial scale structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation thus restricting channel movement for example as the river crosses the purus arch the valley narrows to 20 km compared to an average of 45 km the water surface gradient decreases sediment is deposited and yet the rate of channel migration is negligible hence the effect of the arches is to create a landscape where on the spatial scale of hundreds of kilometres the river is confined and entrenched in its valley is straight and is relatively immobile local valley tilting apparently unrelated to the arch structures also imprints the geomorphology in particular a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large scale oxbow type features on the brazilian amazon river floodplain

spatially variable exhumation rates in orogenic belts an andean example abstract the cordillera real of the bolivian andes is a large tectonically active mountain range that dominates sediment influx into the amazon basin but rates of exhumation in the orogen are poorly known we present 20 new apatite fission track ages from two valleys in the cordillera real to constrain patterns of mountain range exhumation over 106 107 yr we interpret these and previously published data from a third valley using a 2 d thermal model that accounts for topographic and advective influences on measured cooling ages exhumation rates in the cordillera real are 0 2 0 6 mm yr comparable to rates in parts of denali the washington cascades the olympic mountains and the european alps and an order of magnitude slower than rates in taiwan nanga parbat the greater himalaya of nepal and the southern alps of new zealand three to fourfold cooling age variations in the cordillera real imply at least twofold exhumation rate variations within and between valleys over distances of only tens of ki

relation between rock uplift and denudation from cosmogenic nuclides in river sediment in the central alps of switzerland 1 a north south traverse through the swiss central alps reveals that denudation rates correlate with recent rock uplift rates in both magnitude and spatial distribution this result emerges from a study of in situ produced cosmogenic 10be in riverborne quartz in central alpine catchments as a prerequisite we took care to investigate the potential influence of shielding from cosmic rays due to snow glaciers and topographic obstructions to calculate a possible memory from last glacial maximum lgm glaciation and to identify a watershed size that is appropriate for systematic sampling mean denudation rates are 0 27 0 14 mm a for the alpine foreland and 0 9 0 3 mm a for the crystalline central alps the measured cosmogenic nuclide derived denudation rates are in good agreement with post lgm lake infill rates and are about twice as high as denudation rates from apatite fission track ages that record denudation from 9 to 5 ma in general denudation rates are high in areas of high topography and high crustal thickness the similarity in the spatial distribution and magnitude of denudation rates and those of rock uplift rates can be interpreted in several ways 1 postglacial rebound or climate change has introduced a transient change in which both uplift and denudation follow each other with a short lag time 2 the amplitude of glacial to interglacial changes in both is small and is contained in the scatter of the data 3 both are driven by ongoing convergence where their similarity might hint at some form of long term quasi steady state or 4 enhanced continuous quaternary erosion and isostatic compensation of the mass removed accounts for the distribution of present day rock uplift

correction of in situ cosmogenic nuclide production rates for geomagnetic field intensity variations during the past 800 000 years we present integrated relative production rates for cosmogenic nuclides in rock surfaces which take into account reported variations of the geomagnetic field intensity during the past 800 000 yr the calculations are based on the model simulating cosmic ray particle interactions with the earth s atmosphere given by masarik and beer simulation of particle fluxes and cosmogenic nuclide production in the earth s atmosphere j geophys res 104 d10 12099 12111 1999 corrections are nearly independent on altitude between sea level and at least 5000 m the correction factors are essentially identical for all stable and radioactive cosmogenic nuclides with half lives longer than a few hundred thousand years at the equator integrated production rates for exposure ages between 40 000 to 800 000 yr are 10 to 12 higher than the present day values whereas at latitudes 40 geomagnetic field intensity variations have hardly influenced in situ cosmogenic nuclide production correction factors for in situ 14c production rates differ from those of longer lived nuclides they are always smaller than 2 because the magnetic field intensity remained rather constant during the past 10 kyr when the major fraction of the 14c extant today was produced

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

clay mineral composition of river sediments in the amazon basin abstract clay minerals are important in evaluating the maturity of suspended sediments weathering intensity and source area however there are processes that can change the mineral assemblage such as river transportation deposition remobilization and tributary inputs in terms of water discharge and sediment yield the amazon is one of the largest rivers in the world most of the suspended sediments come from the andes crossing the lowlands before reaching the ocean this study measures the spatial distribution of clay mineral assemblages over the entire amazon basin the results obtained show the main features of the amazon river main stem and larger tributaries from their sources to their confluence clay mineral composition highlights the evolution of the madeira and maranon solimoes river which start in the andes with high illite chlorite content downstream smectite contents increase moreover all shield tributaries show high kaolinite content the lower amazon river is characterized by relative high smectite content different from the andean sources the clay mineral results show that suspended sediments of the amazon river have three main sources 1 the andes mountains 2 the amazon shields and 3 the piedmont basins especially the pastaza alluvial megafan and the fitzcarrald arch basin lateral bank erosion plays also a significant role by the introduction of more mature sediments into the river enriched in smectite

exchanges of sediment between the flood plain and channel of the amazon river in brazil sediment transport through the brazilian sector of the amazon river valley a distance of 2010 km involves exchanges between the channel and the flood plain that in each direction exceed the annual flux of sediment out of the river at obidos 1200 mt yr 1 the exchanges occur through bank erosion bar deposition settling from diffuse overbank flow and sedimentation in flood plain channels we estimated the magnitude of these exchanges for each of 10 reaches of the valley and combined them with calculations of sediment transport into and out of the reaches based on sediment sampling and flow records to define a sediment budget for each reach residuals in the sediment budget of a reach include errors of estimation and erosion or deposition within the channel the annual supply of sediment entering the channel from bank erosion was estimated to average 1570 mt yr 1 1 3 the obidos flux and the amount transferred from channel transport to the bars 380 mt yr 1 and the flood plain 460 mt yr 1 in channelized flow 1230 mt yr 1 in diffuse overbank flow totaled 2070 mt yr 1 1 7 the obidos flux thus deposition on the bars and flood plain exceeded bank erosion by 500 mt yr 1 over a 10 16 yr period sampling and calculation of sediment loads in the channel indicate a net accumulation in the valley floor of approximately 200 mt yr 1 over 16 yr crudely validating the process based calculations of the sediment budget which in turn illuminate the physical controls on each exchange process another 300 400 mt yr 1 are deposited in a delta plain downstream of obidos the components of the sediment budget reflect hydrologic characteristics of the valley floor and geomorphic characteristics of the channel and flood plain which in turn are influenced by tectonic features of the amazon structural trough

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

spatially averaged long term erosion rates measured from in situ produced cosmogenic nuclides in alluvial sediment spatially averaged erosion rates of small catchments can be accurately inferred from the concentrations of cosmogenic nuclides in stream sediment here we test this technique at two catchments by comparing erosion rates inferred from cosmogenic nuclides with rates of alluvial fan deposition over the past 16 000 years these two independent estimates agree within one standard error demonstrating that cosmogenic nuclide signatures of stream sediment can be used to measure spatially averaged long term erosion rates using this technique we show that long term erosion rates are an exponential function of average hillslope gradient at these sites

a new estimate of the holocene lowstand level of lake titicaca central andes and implications for tropical palaeohydrology new evidence from piston cores and high resolution seismic reflection data shows that water levels in lake titicaca were as much as 100 m below the present level during the early to mid holocene between 6 and 3 8 14c kyr bp climatological and modelling studies indicate that lake titicaca rainfall depends on convective activity in upwind amazonia the lake level data therefore suggest a drier amazon basin during this time this view is bolstered by an excellent match between the titicaca lake level curve and decreased methane concentrations in greenland ice previously ascribed to drying of low latitude wetlands blunier et al 1995 the postglacial history of lake titicaca fits a global pattern of lake level change in the tropics characterized by opposite phasing between the southern and northern hemispheres this pattern is most likely the result of orbital controls over the intensity of summer insolation

solimoes megashear intraplate tectonics in northwestern brazil a belt of deformation in the solimoes basin of northwestern brazil extends east northeast from near the peruvian border for about 1300 km the belt is characterized by the en echelon arrangement of folds and faults interpreted as the result of right slip displacements in a transpressive regime the structures were formed during late jurassic time probably due to collision of south america with allochthonous terranes as the opening of the south atlantic ocean began the iquitos arch near the western end of the megashear separates the solimoes basin from subandean basins the development of the iquitos arch is interpreted to be related to orogenic loading along western south america by the andean mountain belt in brazil folds and faults associated with the solimoes megashear hold hydrocarbons within paleozoic strata and 15 gas and oil fields have so far been discovered

cosmogenic nuclide evidence for low weathering and denudation in the wet tropical highlands of sri lanka 1 some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep humid tropical highlands of sri lanka the total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites bedrock from mountain crests and bedrock from inselbergs denudation rates are in the range of 5 30 t km 2 yr 1 2 11 mm ky 1 these rates average denudation over the last 50 250 ky weathering exports in rivers draining the mountainous central highlands show that silicate weathering rates are also low varying from 5 to 20 t km 2 yr 1 today 2 7 mm ky 1 but they represent a significant fraction of the total denudation all these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief temperatures and precipitation we speculate that the high relief in sri lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup it is possible that throughout this history hillslopes where not exposing bare bedrock were protected by thick weathered profiles such clay rich layers would inhibit silicate weathering by shielding bedrock from weathering agents in the absence of landscape rejuvenation physical erosion rates are low and fresh mineral surfaces are not being supplied the observation that wet steep tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long term controls of atmospheric co2 budgets high temperature and precipitation which are much invoked though controversial agents for silicate dissolution and co2 drawdown become ineffective in promoting weathering in areas that are not tectonically active

chronological evidence for extension of the jehol biota into southern china abstract the fossils of the cretaceous jehol biota are magnificent exquisitely preserved and extraordinarily diverse strata that may be correlative with the classic jehol localities are known over much of central and eastern china as well as korea japan mongolia and siberia it has been previously hypothesized based on fossil assemblages that the jehol biota progressively expanded from the classic jehol area with younger representatives covering progressively larger geographic ranges it has been interpreted that the youngest jehol biota bearing layers are in the east anhui province of south china here we present robust 40 ar 39 ar dates of 129 0 0 2 ma for one rhyolite sample from the lower longwanshan formation and 127 1 1 3 ma and 128 0 0 9 ma for two volcanic samples from the upper honghuaqiao formation from the outcrops of jehol fossils in east anhui our age results indicate that fossils from these formations are time equivalent to the second phase fossils from the lower yixian formation in the western liaoning ne china and the deposition of these beds is at least 6 ma older than previously estimated therefore our results suggest that the hypothesis of the jehol radiation and migration should be reevaluated

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

luminescence of quartz and feldspar fingerprints provenance and correlates with the source area denudation in the amazon river basin the amazon region hosts the world s largest watershed spanning from high elevation andean terrains to lowland cratonic shield areas in tropical south america this study explores variations in opti

the late quaternary evolution of the negro river amazon brazil implications for island and floodplain formation in large anabranching tropical systems abstract the rio negro has responded significantly in the late pleistocene and holocene to lagged environmental changes largely associated with activity during the last glacial in the amazon basin on the basis of geological structure the rio negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles no deposits of the upper pleniglacial were recognized in the field the oldest recognizable late pleistocene alluvial unit is the upper terrace of middle pleniglacial age ca 65 25 ka reach i tentatively correlated with the oldest terrace identified on the left bank of reach iii at that time the river was mainly an aggradational bed load system carrying abundant quartz sand a product of more seasonal conditions in the upper catchment the late glacial 14 10 ka is represented by a lower finer grained terrace along the upper basin reach i which was recognized in the tiquie curicuriari and vaupes rivers at that time the river carried abundant suspended load as a response to climatic changes associated with deglaciation since about 14 ka the river has behaved as a progradational system infilling in downstream series a sequence of structurally controlled sedimentary basins or compartments creating alluvial floodplains and associated anabranching channel systems reach ii was the first to be filled then reach iii both accumulating mainly sand fine deposits increase downstream in reach iii and become predominant in some anabranch islands of the distal reach the lowermost reaches of the negro v and vi have been greatly affected by a rising base level and associated backwater effect from aggradation of the amazon during late glacial and recent times reach v has acted almost entirely as a fine sediment trap the remarkable anavilhanas archipelago is the product of holocene deposition in the upper part of this sedimentary basin however suspended sediment load declined about 1 5 ka prior to the lower part of this basin becoming infilled the progradational behavior of the rio negro filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones illustrates how a large river system responses to profound changes in late quaternary base level and sediment supply the most stable equilibrium conditions have been achieved in the holocene in reaches iib and iiia where an anabranching channel and erosional relictual island system relatively efficiently convey water and sediment downstream reaches iiib and v never achieved equilibrium conditions during the holocene characterised as they are today with incomplete floodplains and open water

dating sediment burial with in situ produced cosmogenic nuclides theory techniques and limitations abstract dating sediment burial over million year time scales is crucial in many areas of the earth sciences and archeology but is often difficult using traditional techniques sediment burial can be dated by the radioactive decay of cosmogenic nuclides provided that the sediment was exposed to cosmic rays prior to burial dating calculations are straightforward if sediment is buried deeply and rapidly enough to prevent cosmogenic nuclide production after burial however the analysis can be complicated by postburial production if sediment is insufficiently shielded from secondary cosmic ray nucleons and muons this paper discusses how buried sediments can be dated over timescales up to 5 myr using 26 al and 10 be in quartz

modern sedimentation in the lower negro river amazonas state brazil abstract the negro river which flows through the north central amazon basin is one of the largest tributaries of the amazon the name negro comes from the colour of its water which reflects the large quantity of dissolved humic acids and iron oxides that also gives the water its characteristic acid ph the river is estimated to have the fifth largest water discharge in the world about 30 000 m3 s the negro river is characterized by a high dissolved load but a low energy system neotectonics and water quality are the principal factors that control the modern sedimentation in the lower negro river the lower negro river is controlled largely by a nw se tectonic lineament that is a segment of a major tectonic transcurrent dextral megasystem of the amazon basin neotectonism in this area is responsible for the depth of the river and for the occurrence of steep falesias cliffs along some parts of its borders it also seems that neotectonics have influenced the origin of the anavilhanas islands which are a series of anastomosed elongated silty clayey channel bars with internal round or long narrow lakes the igapo which is the forested area flooded during part of the year appears to have a neotectonic origin as well igapos are located on intermediate blocks between the high blocks of the terra firme and the low blocks of the channel the absence of clastic sediments carried in suspension is related to the rare appearance of floodplains which are limited to very thin layers of fine sediments located on the abrasion shelfs carved in clastic deposits of the alter do chao formation sand bars occur in places along the base of the cliffs and along the edges of the channel system these sand bars are composed of quartz sand derived from the reworking of the sand of the alter do chao formation

cratons and orogenic belts of the brazilian shield and their contained gold deposits the brazilian shield extends over much of south america to the east of the andean chain and is partly covered by phanerozoic sedimentary rocks the three major tectonic units older than 900 ma of the shield are the amazon sao francisco and rio de la plata cratons whereas rocks of the neoproterozoic 900 550 ma brasiliano cycle belts surround the cratons in the eastern half of brazil paleoarchean rocks occur as small cratonic nuclei in northeastern brazil but the cratons contain voluminous 3 0 2 6 ga granitic and greenstone belts and a large volume of paleoproterozoic rocks late mesoproterozoic belts 1 2 ga occur in the western and northwestern amazon craton and possibly in northeastern brazil there are voluminous paleoproterozoic and mesoproterozoic intrusive and volcanic rocks in the amazon craton belts of reworked crust are common but the amazon craton grew to the north and west mainly during accretionary orogens the neoproterozoic brasiliano cycle belts contain 5 of juvenile crust in central and southern brazil but are dominantly derived from reworking of older crust it is possible that the cratons were continuous in a super continent during the mesoproterozoic prior to the brasiliano cycle gold deposits are concentrated in the archean and paleoproterozoic terranes including the carajas and quadrilatero ferrifero gold provinces of the amazon and sao francisco cratons respectively but some also occur in younger terranes recent alluvial gold deposits and supergene deposits in the weathering zone are widespread in brazil

a test of the cosmogenic 10be meteoric 9be proxy for simultaneously determining basin wide erosion rates denudation rates and the degree of weathering in the amazon basin we present an extensive investigation of a new erosion and weathering proxy derived from the 10be meteoric 9be stable ratio in the amazon river basin this new proxy combines a radioactive atmospheric flux tracer meteoric cosmogenic 10be with 9be a trace metal released by weathering results show that meteoric 10be concentrations 10be and 10be 9be ratios increase by 30 from the andes to the lowlands we can calculate floodplain transfer times of 2 30 kyr from this increase intriguingly however the riverine exported flux of meteoric 10be shows a deficit with respect to the atmospheric depositional 10be flux most likely the actual area from which the 10be flux is being delivered into the mainstream is smaller than the basin wide one despite this imbalance denudation rates calculated from 10be 9be ratios from bed load suspended sediment and water samples from amazon rivers agree within a factor of 2 with published in situ 10be denudation rates erosion rates calculated from meteoric 10be measured from depth integrated suspended sediment samples agree with denudation rates suggesting that grain size induced variations in 10be are minimized when using such sampling material instead of bed load in addition the agreement between erosion and denudation rates implies minor chemical weathering intensity in most amazon tributaries indeed the be specific weathering intensity calculated from mobilized 9be comprising reactive and dissolved fractions that are released during weathering is constant at approximately 40 of the total denudation from the andes across the lowlands to the amazon mouth therefore weathering in the amazon floodplain is not detected

new geological framework for western amazonia brazil and implications for biogeography and evolution abstract although many of the current hypotheses to explain the origin and distribution of the amazon biodiversity has been based directly or indirectly on geological data the reconstruction of the geological history of the amazon region is still inadequate to analyze its relationship with the biodiversity this work has the main goal to characterize the sedimentary successions formed in the brazilian amazon in the neogene quaternary discussing the evolution of the depositional systems through time and analyzing their main controlling mechanisms in order to fill up this gap radar image interpretation sedimentological studies and radiocarbon dating allowed the mapping of plio pleistocene to holocene units along the solimoes amazonas river brazil this integrated work led to the characterization of five sedimentary successions overlying miocene deposits of the solimoes pebas formation which include the following ica formation plio pleistocene deposits q1 37 400 43 700 14c yr b p deposits q2 27 200 14c yr b p deposits q3 6730 2480 14c yr b p and deposits q4 280 130 14c yr b p these deposits occur mostly to the west of manaus forming nw se elongated belts that are progressively younger from sw to ne indicating a subsiding basin with a depocenter that migrated to the ne the reconstruction of the depositional history is consistent with significant changes in the landscapes hence the closure of a large lake system at the end of the miocene gave rise to the development of a plio pleistocene fluvial system this was yet very distinct from the modern drainage with shallow energetic highly migrating braided to anastomosed channels having an overall northeast outlet this fluvial system formed probably under climatic conditions relatively drier than today s during the early pleistocene there was pronounced erosion followed by a renewed depositional phase ca 40 000 14c yr b p with the development of prograding lobes and or crevasse splays associated with a lake system i e fan delta and or fluvial flood plain areas after a period of erosion a fluvial system with eastward draining channels started to develop at around 27 000 14c yr b p the fluvial channels were overflooded in mid holocene time this flooding is attributed to an increased period of humidity with a peak between 5000 and 2500 14c yr b p the data presented herein support that rather than being a monotonous area the amazonia was a place with frequent changes in landscape throughout the neogene quaternary probably as a result of climatic and tectonic factors we hypothesize that these changes in the physical environment stressed the biota resulting in speciation and thus had a great impact on modern biodiversity

earth surface erosion and weathering from the 10be meteoric 9be ratio abstract the isotope ratio of the meteoric cosmogenic nuclide 10 be to the mineral derived stable isotope 9 be discloses both the earth surface denudation rate and its weathering intensity we develop a set of steady state mass balance equations that describes this system from a soil column over the hillslope scale to an entire river basin the prerequisites making this new approach possible are 1 the 9 be concentration of parent rock typically 2 5 0 5 ppm in granitic and clastic sedimentary lithologies is known 2 both be isotopes equilibrate between the fluids decomposing rock and reactive solids formed during weathering and 3 a critical spatial scale is exceeded at which the fluxes of both isotopes into and out of the weathering zone are at steady state over the time scale of weathering typically 10 kyr for these cases the isotope ratios can be determined in bulk sediment or soil on leachates from the reactive adsorbed and pedogenic mineral bound phase in sediment or soil and even on the dissolved phase in river water the 10 be 9 be ratio offers substantial advantages over the single isotope system of meteoric 10 be the latter system allows to directly determine erosion rates only in the case that 10 be is fully retentive in the weathering zone and that riverine sorting has not introduced grain size dependent 10 be concentration gradients in sediments we show the feasibility of the 10 be 9 be tracer approach at the river scale for sediment and water samples in the amazon basin where independent estimates of denudation rates from in situ produced 10 be exist we furthermore calculate meaningful denudation rates from a set of published 10 be 9 be ratios measured in the dissolved load of globally distributed rivers we conclude that this isotope ratio can be used to reconstruct global paleo denudation from sedimentary records

sediment budget of the napo river amazon basin ecuador and peru the upstream downstream sediment budget along the napo river 100 520 km2 6300 m3 s 1 was studied in the andean foothills of ecuador at the west of the amazon basin a comparative study was made during four hydrological cycles 2001 2005 for three hydrological stations located upstream and during one hydrological cycle 2004 2005 for the fourth one located near the mouth of the napo river region of iquitos in peru this analysis showed an unusual increase in the concentration of suspended sediment recorded for the western part of the amazon plain like the runoff 81 l s 1 km2 which is a world s maximum the erosion rate 1160 t km 2 year 1 i e 47 of total suspended solid tss export at the exit of ecuador one of the highest for a floodplain basin is the result of a stepper slope than in the rest of the andean foothills where typically sedimentation phenomena are predominant and can be explained in part by a greater tectonic activity similar phenomenes were evidenced in small mountainous rivers in new guinea milliman and syvitski 1992 milliman 1995 on the headwaters of the napo river drainage basin the tectonic uplift causes the pastaza megafan s existence this progressively diverts the course of napo river towards north and also provokes the remobilization of fine fluvial deposits moreover this geodynamic trend is completed by the impact of volcanic eruption earthquakes and landslides the combination of these phenomena so common in the region has provided a large sediment transfer not only at present but also in the past as can be confirmed by the presence of incised terraces mainly formed by volcanic materials then these results were compared with a similar study carried out further south in the madeira basin at the bolivian foothills these studies show the spatio temporal variability of the relation between sediment transfer and geodynamic processes at the andean piedmont copyright 2009 john wiley sons ltd