paleovegetation simulations of lowland amazonia and implications for neotropical allopatry and speciation paleovegetation modeling simulations of the lowland amazon basin were made to assess the relative importance of glacial climate and atmospheric co2 for altering vegetation type and structure as well as to explore the potential physiological mechanisms underlying these ecosystem level responses modeling results support the view that widespread invasion of grasslands into the amazon lowlands during the last glaciation was not likely glacial cooling was probably responsible for maintaining glacial forest cover via its effects in reducing photorespiration and decreasing evapotranspiration which collectively improve plant carbon and water relations modeling results confirm that leaf area index lai a common proxy for canopy density is highly sensitive to independent and interactive changes in climate and low concentration of atmospheric co2 and the results show considerable region to region variation during the last glaciation heterogeneous variations in glacial vegetation lai may have promoted allopatric speciation by geographically isolating species called vicariance in the forest sub canopy the proposed vicariance hypothesis incorporating spatial variations in canopy density conforms to many of the essential tenets addressed by previous neotropical speciation models but also helps to overcome some of their inconsistencies c 2001 university of washington

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

applications of organic geochemistry to paleolimnological reconstructions a summary of examples from the laurentian great lakes the organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions evaluate histories of climate change and assess impacts of humans on local ecosystems the elemental isotopic and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area and they serve as proxies of organic matter delivery and accumulation sedimentary records from the north american great lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions the records of these lakes date from retreat of the laurentide ice sheet around 12 ka include the mid holocene hypsithermal and show consequences of recent human changes low corg ntotal ratios indicate that most of the sediment organic matter in the great lakes is from algal production yet changes in biomarker molecule compositions also show that varying amounts of land plant organic matter have been delivered to the lakes elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13c values in the organic matter of sediments that were deposited in the 1960s and 1970s increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the great lakes accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the great lakes emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising yet some potentially important measurements remain underutilized

implications for intraplate volcanism and back arc deformation in northwestern new zealand from joint inversion of receiver functions and surface waves summary we employ a joint inversion of teleseismic receiver functions and surface wave phase velocities to determine the shear wave velocity structure in the crust and upper mantle beneath northwestern new zealand receiver functions primarily contain information on velocity contrasts while surface waves are sensitive to the average shear velocity with depth by performing a joint inversion we reduce the limitations of each method resulting in a more robust shear wave model inversion results reveal regions of low shear wave velocity of 2 8 km s 1 in the mid crust 10 19 km depth and 4 0 km s 1 in the upper mantle 70 90 km depth beneath quaternary intraplate basalt fields we infer that the mid crustal low velocity zones lvzs are bodies of partial melt most likely rhyolite intrusions we suggest that the upper mantle lvz is caused by the melt producing regions of the upper mantle and is a source for the basalts of the auckland volcanic field this is in agreement with models for a shallow upper mantle source rather than a deep seated mantle plume for the auckland volcanism average shear velocities for the upper crust are 3 4 3 6 km s 1 increasing to 3 6 4 0 km s 1 in the lower crust the moho is interpreted to be 29 1 km deep in the southern end of the array shallowing to 26 1 km towards the edge of the continental shelf of northern new zealand low upper mantle shear velocities of 4 2 0 1 km s 1 are observed and are thought to represent a small percentage of partial melt in the upper mantle beneath this back arc region these low velocities extend to greater depth 90 km beneath the southern stations indicating that mantle deformation associated with the mantle wedge to the south may extend as far north as the southern end of our array near surface low velocity layers extend to 6 km depth beneath a station at the northern tip of new zealand likely representing the expression of a thick sequence of sediments from the northland allochthon and the more recent northland basin

instability of a lithospheric step beneath western north island new zealand lithospheres of different thicknesses are often juxtaposed by movement on a continental transform boundary such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle here we show for plausible values of the lithospheric viscosity a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere as the mantle deforms it also progressively thickens and then thins the overlying crust creating a topographic wave that migrates in concert with the removal of mantle lithosphere within western north island new zealand geophysical data define a sharp lithospheric step across the taranaki ruapehu line and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m y the rate of observed north to south migration of the wave 30 mm yr its wavelength 250 km and amplitude 1 km are compatible with it being caused by progressive removal of mantle lithosphere if the viscosity of the uppermost lithospheric mantle is 5 10 20 pa s providing one of the clearest examples yet of this fundamental geological process

late glacial cooling in amazonia inferred from pollen at lagoa do caco northern brazil new pollen data from a core at lagoa do caco maranhao state brazil 2 58 s 43 25 w 120 m elevation show higher frequencies of podocarpus at the end of the pleistocene than today the increase in podocarpus which follows the successive increase of various pioneer species such as didymopanax melastomataceae combretaceae and cecropia implies a progressive late glacial increase of moist and cool climatic conditions a comparable increase in podocarpus is found in other lowland records in amazonia a review of published pollen data from amazonia suggests that the moisture source was from the southeast by contrast present day moisture comes from the tropical atlantic and from the amazon basin with its convective precipitation the likely cause for the southeastern moisture source between ca 15 000 and 14 500 cal yr b p was enhanced polar antarctic advection that reached low latitudes and maintained year round the meteorological equator in its austral winter position at northern latitudes or reduced drastically its southward summer displacement this hypothesis is consistent with marine and ice core records

amazonia during the last glacial abstract new data on the vegetational history and quaternary geology of amazonia permit an improved reconstruction of past environments in amazonia during the last glacial period although limited data from rondonia carajas and guyana show that in certain areas savanna type vegetation and savanna forest had replaced the rain forest during the late pleniglacial ca 22 000 13 000 yr b p the amazonian forest may have been split up into one major west amazonian and several other medium size forest areas this suggests a decline in rainfall of 500 to 1000 mm a reduction of 25 to 40 temperatures may also have been 2 to 6 c 4 2 c lower than today possibly substancially influencing amazonian vegetation during the humid middle pleniglacial 55 000 26 000 yr b p rivers carried a lot of water and sediment resulting in the deposition of lower terrace sediments with one dry interruption around 40 000 yr b p carajas and katira in carajas there is evidence of dry periods ocurring at about 40 000 yr b p and during the early pleniglacial ca 60 000 yr b p rivers carried little water and incised into the low terrace sediments during the dry late pleniglacial water levels rose during the late glacial 13 000 10 000 yr b p or at the beginning of the holocene 10 000 yr b p sedimentation in and of the present inundation valleys commenced after that

results confirm a relatively dry setting during the last glacial mis 3 and lgm in carajas amazonia a comment on guimaraes et al recent pollen rain data from lake amendoim in carajas southeastern amazonia revealed a dominant forest formation spectrum where locally montane savannas occupy a larger proportion of the lake s

observations on late pleistocene cooling and precipitation in the lowland neotropics although accurate reconstructions of amazonian palaeoclimates are central to under standing the distribution and history of neotropical biodiversity current reconstructions based on proxy data are discordant and subject to intense debate we review some current thinking in amazonian climatology and incorporate some new ideas in an attempt to explain the apparently con tradictory records of palaeoprecipitation from across the amazon basin we suggest that palaeoecol ogists need to recognise that organised convective radiation the process that induces most wet season rainfall in amazonia should be treated as a phenomenon related to but separate from the passage of the inter tropical convergence zone itcz decoupling itcz migration from larger con vective activity provides a mechanism to account for observed palaeoclimatic heterogeneity patterns of observed precipitation change are consistent with the long term persistence of closed forest across much of amazonia indicating that the greatest changes in precipitation during the last glacial max imum came during the wet season which would have little negative impact on forest extent neo tropical cooling at the last glacial maximum lgm is widely accepted although the estimates of that cooling range between 1 c and 5 c in answering the basic question what is meant by cooling we observe that interhemispheric ice mass asymmetry may have caused cooling to be manifested differently according to location a terrestrial cooling of ca 5 c as well as radiative cooling and event based cooling combined to induce vegetation change probably both absolute temperature and mean monthly minima were reduced by polar air incursions in the northern neotropics how ever in the southern neotropics mean monthly minima were reduced by more frequent incursions of patagonian air masses but absolute minima may have been largely unchanged copyright 2004 john wiley sons ltd

results from the mars global surveyor thermal emission spectrometer the thermal emission spectrometer spectra of low albedo surface materials suggests that a four to one mixture of pyroxene to plagioclase together with about a 35 percent dust component provides the best fit to the spectrum qualitative upper limits can be placed on the concentration of carbonates 10 percent olivine 10 percent clay minerals 20 percent and quartz 5 percent in the limited regions observed limb observations in the northern hemisphere reveal low lying dust hazes and detached water ice clouds at altitudes up to 55 kilometers at an aerocentric longitude of 224 degrees a major dust storm developed in the noachis terra region the south polar cap retreat was similar to that observed by viking

models of the upper mantle beneath the central north island new zealand from speeds and anisotropy of subhorizontal p waves pn 1 the central north island of new zealand shows significant variations in pn wave speeds over small distances ranging from 8 5 0 2 km s to 7 4 0 1 km s over a distance of 150 km a combination of national network seismometers local volcanic seismic monitoring networks and temporary deployments are used to collect arrival times from local events during the period of 1990 2006 the data set consists of approximately 11 200 pn observations from 3000 local earthquakes at 91 seismograph sites we have created a method that allows us to model the predominant wavelength features of p wave speeds in the uppermost mantle as well as estimating values of mantle anisotropy and irregularities in the crust beneath stations using least squares collocation the resulting model shows distinct variations in uppermost mantle pn velocities velocities of less than 7 5 km s are found beneath the back arc extension region of the central volcanic region and under the taranaki volcanic region indicating the presence of water and partial melt the region to the east shows extremely high velocities of 8 3 8 5 km s where the p waves are traveling within the subducting pacific slab slightly lower than normal mantle velocities of 7 8 8 1 km s are found in the western north island suggesting a soft mantle pn anisotropy estimates throughout the north island show predominately trench parallel fast directions ceasing to nulls in the west anisotropy measurements indicate the strain history of the mantle null anisotropy measurements suggest an undisturbed mantle suggesting that mantle beneath the western north island is young

detection of mantle earthquakes beneath the east african rift summary recordings from a temporary seismic network in the rwenzori region of the east african rift show seven earthquakes at a depth range between 53 and 60 km evidence from receiver functions indicates a crustal thickness of about 32 km in this region suggesting that the earthquakes are located well within the mantle beneath the rift the occurrence of faulting at this depth is of relevance for models of continental rift initiation and related deep magmatic processes

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

response of the amazon rainforest to late pleistocene climate variability abstract variations in amazonian hydrology and forest cover have major consequences for the global carbon and hydrological cycles as well as for biodiversity yet the climate and vegetation history of the lowland amazon basin and its effect on biogeography remain debated due to the scarcity of suitable high resolution paleoclimate records here we use the isotopic composition d and 13 c of plant waxes from a high resolution marine sediment core collected offshore the amazon river to reconstruct the climate and vegetation history of the integrated lowland amazon basin for the period from 50 000 to 12 800 yr before present our results show that d values from the last glacial maximum were more enriched than those from marine isotope stage mis 3 and the present day we interpret this trend to reflect long term changes in precipitation and atmospheric circulation with overall drier conditions during the last glacial maximum our results thus suggest a dominant glacial forcing of the climate in lowland amazonia in addition to previously suggested thermodynamic mechanisms of precipitation change which are directly related to temperature we conclude that changes in atmospheric circulation are crucial to explain the temporal evolution of amazonian rainfall variations as demonstrated in climate model experiments our vegetation reconstruction based on 13 c values shows that the amazon rainforest was affected by intrusions of savannah or more open vegetation types in its northern sector during heinrich stadials while it was resilient to glacial drying this suggests that biogeographic patterns in tropical south america were affected by heinrich stadials in addition to glacial interglacial climate variability

a review of late quaternary inland dune systems of south america east of the andes abstract inland dune systems of south america occur in a variety of environmental settings under different climatic conditions from humid subhumid to semiarid arid this contribution provides the state of the art on dune research of inland systems east of the andes tropical northern 10 n 10 s including the dune fields of llanos del orinoco and the amazonian basin are mainly formed from parabolic blowout or deflation basins and linear dunes developed on river floodplains aeolian activity was reported to occur near the late pleistocene holocene boundary in llanos del orinoco over the last 17 ka in the branco river basin and 32 6 ka in the negro river basin both in the amazonian region in general the attributed aeolian origin is controversial the morphology difficult to determine and the chronology still extremely limited and rather debatable in some cases the southern tropical region 10 s 22 s includes aeolian systems situated across the extensive lowlands of the gran chaco and those located in fluvial basins related to the cratonic brazilian shield sao francisco river pantanal and mato grosso the oldest barchanoid barchan ridges at the gran chaco were dated 36 33 ka and subsequent aeolian sand accumulation occurred at 18 ka 14 12 ka 10 9 ka and mid to late holocene times parabolic dunes at the sao francisco river floodplain show an oldest episode generated between 28 and 11 ka followed by eolian activity during the early to mid holocene no chronological data is available for the linear and lunette dunes described at the pantanal linear ridges were distinguished in the upper parana river mato grosso with associated sand accumulation at different intervals of the holocene the last reviewed region is the andean piedmont and western pampas at subtropical eastern south america comprising a large variety of dune morphologies barchan barchanoid linear parabolic transverse and lunette dunes deflation basins and sand sheets luminescence ages suggest the occurrence of aeolian activity intervals during late glacial times and the holocene the general understanding on the dynamic of south american dune systems is hampered by the paucity of chronological information and the representativeness of some dune records whether they reflect local or regional environmental conditions is still a matter of discussion in several areas that need further studies

evidence for deep melting of hydrous metasomatized mantle pliocene high potassium magmas from the sierra nevadas 1 phase equilibrium experiments have been conducted on a primitive pliocene olivine leucitite wc 1 from the central sierra nevada california the near liquidus phase relations were determined from 1 2 to 3 4 gpa and at temperatures from 1350 to 1460 c in a piston cylinder apparatus the composition with 2 h 2 o is multiply saturated with olivine and clinopyroxene at approximately 3 1 gpa and 1460 c and with 6 water in the coexisting melt phlogopite is stable these results indicate that the magma was derived from a hydrous source at greater than 100 km depth xenoliths carried by other young pliocene lavas in the vicinity of wc 1 have yielded temperatures of equilibrium from 700 to 900 c with one outlier at 1060 c these xenoliths are consistent with the hypothesis that the lower lithosphere under the sierra nevada delaminated just prior to the pliocene and fluid metasomatized mantle melted to produce the high potassium pliocene lavas we suggest that subduction derived fluids drive a reaction that consumes garnet orthopyroxene to create clinopyroxene phlogopite and that the high potassium sierran magmas are created by melting phlogopite clinopyroxene metasomatized peridotite

late pleistocene glacial forest of humaita western amazonia abstract glacial aged vegetation dynamics of the humaita western brazilian amazonia were studied by pollen sedimentary facies 14 c dating 13 c org and c n molar two sediment cores were taken to a depth of 10 and 8 m from areas covered by grassland and dense open forest respectively the deposits represent a succession of sediment accumulation in active channel 42 600 cal yr b p abandoned channel floodplain 42 600 to 39 000 cal yr b p and oxbow lake sedimentary environments 39 000 cal yr b p to modern the predominance of mud sediments depletion of 13 c org and decrease in c n molar values identify the lake establishment in these settings low energy subaqueous conditions were developed locally favoring preservation of a pollen assemblage representing herbaceous vegetation some modern taxa from amazonia and cold adapted plants from the andes represented by alnus 2 11 hedyosmum 2 17 weinmannia 0 18 podocarpus 0 4 ilex 0 4 and drymis 0 1 at least between 42 600 and alnus with other cold adapted plants from the andes during the late pleistocene indicates that alnus probably penetrated the western amazonia lowland or was growing closer to the study site due to cooler temperatures during glacial times the present study presents the first report of a glacial age forest containing alnus in areas of the brazilian amazonian lowlands in addition to its palaeogeographical importance this work demonstrates the effectiveness of using a combination of proxies for reconstructing sedimentary environments associated with vegetation

amazonian paleoecological histories one hill three watersheds abstract data from the hill of six lakes in the northwestern brazilian amazon region provide three records of paleoclimatic and vegetation change in lowland amazonia that span the last 170 000 years three lakes verde pata and dragao which occupy separate watersheds on the hill provide the most detailed image yet obtained of ice age conditions in lowland amazonia well dated sedimentary records for fossil palynological charcoal cation and pigment data are presented the data indicate the continuous presence of mesic forest throughout the last ice age oscillations of lake level are recorded and the lowstands are attributed to reduced precipitation inputs to systems delicately balanced between water loss leakage through the floor of the basin and gain precipitation gross stratigraphy algal remains and paleochemistry suggest that the stands of high and low lake level were cyclic apparently correlating precessional orbital variation times of lake lowstand coincide with wet season december january february djf insolation minima the strongest of eight lowstand cycles occurred ca 35 000 to 27 000 cal bp even during lowstand episodes pollen is well preserved and provides a clear signal of uninterrupted forest cover the principal lowland elements are continuously present in the record suggesting the long term quaternary availability of the lowland forest biome in this region however during the late pleistocene the forest changed in composition with the expansion or invasion of montane floral elements creating communities of the mesic forest biome without modern analogs while precipitation cycles were causing lake levels to rise and fall the principal influence on vegetation appears to have been cooling in the late pleistocene the population expansion of montane elements at lower elevations is consistent with a cooling of 4 5 c

palaeoecology and climate of the amazon basin during the last glacial cycle our interpretation of the available palynological data from the amazon basin is that they show a long history of continuous occupation by forest we have thoroughly reviewed the accumulated data leading to this conclusion elsewhere but briefly recapitulate the arguments here one implication of this botanical reconstruction if correct is that interpretations of geomorphological features requiring an arid land surface for glacial age amazonia require reassessment copyright 2000 john wiley sons ltd

reconstructing habitats in central amazonia using megafauna sedimentology radiocarbon and isotope analyses abstract a paleomegafauna site from central amazonia with exceptional preservation of mastodons and ground sloths allows for the first time a precise age control based on 14 c analysis which together with sedimentological and 13 c isotope data provided the basis to discuss habitat evolution within the context of climate change during the past 15 000 yr the fossil bearing deposits trapped within a depression in the paleozoic basement record three episodes of sedimentation formed on floodplains with an intermediate unit recording a catastrophic deposition through debris flows probably favored during fast floodings the integrated approach presented herein supports a change in humidity in central amazonia through the past 15 000 yr with a shift from drier to arboreal savanna at 11 340 50 14 c yr b p and then to a dense forest like we see today at 4620 60 14 c yr b p

a bayesian approach to determining and parametrizing earthquake focal mechanisms summary we develop a new probabilistic bayesian method for estimating the distribution of focal mechanism parameters based on seismic wave polarity data we investigate the use of generalized matrix fisher distributions for parametrizing focal mechanism uncertainties the advantages of our approach are that it 1 models the data generation process and incorporates observational errors particularly those arising from imperfectly known earthquake locations 2 allows exploration of the entire parameter space 3 leads to natural point estimates of focal mechanism parameters 4 allows the inclusion of a priori information about the focal mechanism parameters and 5 allows the resulting posterior pdf to be well approximated by generalized matrix fisher distributions we present here the results of our method in two situations we first consider the case in which the seismic velocity of the region of interest described by a velocity model is presumed to be precisely known with application to seismic data from the raukumara peninsula new zealand we then consider the case in which the velocity model is imperfectly known with application to data from the kawerau region new zealand we find that our estimated focal mechanism solutions are for the most part consistent with all available polarity data and correspond closely to solutions obtained using established methods further the generalized matrix fisher distributions we examine provide a good fit to our bayesian posterior pdf of the focal mechanism parameters finally we demonstrate how informative prior distributions on focal mechanism parameters can be incorporated into our model

upper quaternary marine terrace chronology and deformation south taranaki new zealand an extensive flight of late quaternary marine terraces in the south taranaki region of the north island of new zealand is dated using fission track radiocarbon and amino acid racemization methods a simple model of terrace deformation based on a doming type uplift pattern allows terrace strandline ages to be estimated twelve strandlines are recognized and are estimated to have formed during high sea level stands at 60 000 80 000 100 000 120 000 135 000 210 000 310 000 340 000 400 000 450 000 520 000 600 000 and 680 000 yr b p

hydroclimate changes across the amazon lowlands over the past 45 000 years reconstructing the history of tropical hydroclimates has been difficult particularly for the amazon basin one of earth s major centres of deep atmospheric convection for example whether the amazon basin was substantially drier or remained wet during glacial times has been controversial largely because most study sites have been located on the periphery of the basin and because interpretations can be complicated by sediment preservation uncertainties in chronology and topographical setting here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide our results are based on a decadally resolved uranium thorium dated oxygen isotopic record for much of the past 45 000 years obtained using speleothems from paraiso cave in eastern amazonia we interpret the record as being broadly related to precipitation relative to modern levels precipitation in the region was about 58 during the last glacial maximum around 21 000 years ago and 142 during the mid holocene epoch about 6 000 years ago we find that as compared with cave records from the western edge of the lowlands the amazon was widely drier during the last glacial period with much less recycling of water and probably reduced plant transpiration although the rainforest persisted throughout this time

asymmetric delamination and convective removal numerical modeling comparison with evolutionary models for the alboran sea region convective removal and mantle delamination are geodynamical mechanisms proposed to explain the presence of extension in the alboran sea within a regional context of compression using a new thermomechanical algorithm we present here a quantitative evaluation and comparison of conceptual models based on these geodynamical mechanisms in contrast to the in situ convective removal process the laterally propagating delamination mechanism is shown here to be consistent with first order features of the alboran sea such as the thinning thickening distribution intermediate depth seismicity and upper mantle structure imaged by seismic tomography the lower crust is predicted to reach depths of 100 150 km in some areas due to mechanically driven viscous drag of the downwelling mantle

paleo environmental change in amazonian and african rainforest during the lgm abstract the paper provides new and comparative insight into the ecological history of the two largest continental tropical forest areas during the last glacial maximum lgm the tropical forest regions are of particular interest because they present a large source of heat and have been shown to have significant impact on the extra tropical atmospheric circulation they are also the most intense land based convective centers thus especially from the tropics paleoecological information is needed as benchmarks for climate modeling the african data for lgm climates were published earlier including the reconstructed paleoprecipitation patterns deduced from ssts the tropical south american lgm data were interpreted from pollen geochemical and 18o stable oxygen isotope data from brazil and selected surrounding areas the available terrestrial data are consistent with the sst derived precipitation data for the tropical forests in brazil and for africa however the impact of lgm climate extremes was less severe in the amazon than in the congo basin the lgm humid forest area including evergreen and semi deciduous forest types in africa was probably reduced by 84 in contrast the amazon humid forest area probably shrank to 54 of their present day extension still there are different interpretations with respect to the amount of reduction of the amazon forest area during the lgm although direct information about lgm climates in amazonia is still limited the more detailed map obtained in the present work however allows a more reliable characterization of the last glacial tropical environment than previously published for the amazon region

subduction evolution and mantle dynamics at a continental margin central north island new zealand 1 central north island new zealand provides an unusually complete geological and geophysical record of the onset and evolution of subduction at a continental margin whereas most subduction zones are innately two dimensional north island of new zealand displays a distinct three dimensional character in the back arc regions specifically we observe mariana type subduction in the back arc areas of central north island in the sense of back arc extension high heat flow prolific volcanism geothermal activity and active doming and exhumation of the solid surface evidence for emplacement of a significant percent of new lithosphere beneath the central north island comes from heat flux of 25 mw km of strike of volcanic zone and thinned crust underlain by rocks with a seismic wave speed consistent with underplated new crust seismic attenuation qp 1 is high 240 and rhyolitic and andesitic volcanism are widespread almost complete removal of mantle lithosphere is inferred here in pliocene times on the basis of the rock uplift history and upper mantle seismic velocities as low as 7 4 0 1 km s in contrast southwestern north island exhibits chilean type back arc activity in the sense of compressive tectonics reverse faulting low heat flow thickened lithosphere and strong coupling between the subducted and overriding plates this rapid switch from mariana type to chilean type subduction occurs despite the age of the subducted plate being constant under north island moreover stratigraphic evidence shows that processes that define the extensional back arc area the central volcanic region are advancing southward into the compressional system wanganui basin at about 10 mm yr we link the progression from one system to another to a gradual and viscous removal of thickened mantle lithosphere in the back arc regions thickening occurred during the miocene within the taranaki fault zone the process of thickening and convective removal is time and temperature dependent and has left an imprint in both the geological record and geophysical properties of central north island which we document and describe

late quaternary of the middle caqueta river area colombian amazonia the late quaternary history of the middle caqueta river area in colombia northwestern amazonia is described based on observations of river bank sections radiocarbon dates and palynological analyses of organic layers in floodplain and low terrace sediments of the caqueta river it is shown that the late pleistocene and holocene climatic changes that took place in the andean cordilleras were related to the depositional and erosional history of the caqueta river in the colombian amazonian lowlands the low terrace sediments consist of sandy and gravelly deposits covered by clays that sometimes contain lenses of peaty material from these organic low terrace sediments seven finite radiocarbon dates were obtained of middle pleniglacial age between 56 000 and 30 000 yr bp the coarse textured basal deposits of the low terrace apparently stem from the early part of the middle pleniglaciai period during which the effective rainfall in the andes was relatively high and the andean glaciers had a considerable extension palynological data from silty sediments with organic remains at one site show an interval when drier and more open types of vegetation on poor soils must have covered a larger area than today but amazonian forest was still the dominating type of vegetation this interval might correspond to one of the middle pleniglacial savanna intervals from eastern amazonia carajas no organic sediments from the upper pleniglacial period were found and hence radiocarbon dates were not obtained in the andes this period had a very cold climate with low effective rainfall and in the east amazonian carajas area it is characterised by the relative extension of open savanna vegetation the river run off and sediment transport must have been much lower than in the middle pleniglacial and the caqueta river cut itself down in its own sediments two late glacial radiocarbon datings obtained at one site ca 12 500 yr bp indicate the existence of a late glacial sedimentation phase separated from the holocene sequence by a minor erosional phase organic layers in the holocene floodplain sediments yielded 28 radiocarbon dates between 10 000 and 355 yr bp holocene sedimentation started with the rapid deposition of sandy clay possibly in a partly permanently inundated caqueta valley during the major part of the holocene silty clays were deposited with a dominant seasonal inundation cycle

nature and causes of quaternary climate variation of tropical south america abstract this selective review of the quaternary paleoclimate of the south american summer monsoon sasm domain presents viewpoints regarding a range of key issues in the field many of which are unresolved and some of which are controversial 1 el nino southern oscillation variability while the most important global scale mode of interannual climate variation is insufficient to explain most of the variation of tropical south american climate observed in both the instrumental and the paleoclimate records 2 significant climate variation in tropical south america occurs on seasonal to orbital i e multi millennial time scales as a result of sea surface temperature sst variation and ocean atmosphere interactions of the tropical atlantic 3 decadal scale climate variability linked with this tropical atlantic variability has been a persistent characteristic of climate in tropical south america for at least the past half millennium and likely far beyond 4 centennial to millennial climate events in tropical south america were of longer duration and perhaps larger amplitude than any observed in the instrumental period which is little more than a century long in tropical south america these were superimposed upon both precession paced insolation changes that caused significant variation in sasm precipitation and eccentricity paced global glacial boundary conditions that caused significant changes in the tropical south american moisture balance as a result river sediment and water discharge increased and decreased across tropical south america lake levels rose and fell paleolakes arose and disappeared on the altiplano glaciers waxed and waned in the tropical andes and the tropical rainforest underwent significant changes in composition and extent to further evaluate climate forcing over the last glacial cycle 125 ka we developed a climate forcing model that combines summer insolation forcing and a proxy for north atlantic sst forcing to reconstruct long term precipitation variation in the sasm domain the success of this model reinforces our confidence in assigning causation to observed reconstructions of precipitation in addition we propose a critical correction for speleothem stable oxygen isotopic ratios which are among the most significant of paleoclimate proxies in tropical south america for reconstruction of variation of paleo precipitation or sasm intensity however it is already well known that any particular 18o value observed in speleothem carbonate is affected by two processes that have nothing to do with changes in precipitation amount the influence of temperature on carbonate water isotopic fractionation in the cave and the influence of changing 18o of seawater quantitatively accounting for both artifacts can significantly alter the interpretations of speleothem records in tropical south america both adjustments act in the same direction and have the tendency to increase the true amplitude of the paleo hydrologic signal but by different amounts in glacial and inter glacial stages these corrections have even graver implications for the interpretation of tropical northern hemisphere speleothem records e g chinese speleothems where the combined adjustments tend to decrease or even eliminate the true signal amplitude

palaeofires in amazon interplay between land use change and palaeoclimatic events interpreting the geological record of amazon biomass combustion requires comparing charcoal accumulation rates in various biomes at different time scales charcoal accumulation rates a proxy for palaeofire records were obtained in sediment cores from amazon lakes surrounded by several vegetation types and from a reservoirs in an intense land use change region the records presented in this study were obtained in the following areas i a reservoirs in alta floresta region northern mato grosso state ii lago do saci southern para state a lake close to alta floresta and located at the southern border of para state iii a bog in an ecotone area in the humaita region southern amazonas state iv lakes in lateritic iron crust of the carajas hills southeastern para state v lago comprido a floodplain lake close to the amazon river and surrounded by tropical rain forest monte alegre para state vi lagoa da pata in the morro dos seis lagos alkaline complex sao gabriel da cachoeira amazonas state and vii lago caracarana a secluded lake in the northern amazon cerrado roraima state the highest charcoal accumulation rates were observed for modern records related to an intense change in land use at alta floresta which had no precedent during the holocene history of the amazon high charcoal accumulation rates that were observed in the carajas region during low lake level phases in the amazon in the mid holocene were comparable to those at the onset of the human settlement in alta floresta region an increase in charcoal accumulation rate was observed in the late holocene when the lake level was high suggesting an interaction between climates and human presence low charcoal accumulation rates are typical of modern high rainfall environments as observed in lagoa da pata where the environment is not susceptible to occurrences of wildfires even during relatively drier climatic phases low charcoal accumulation rates also exist in the relatively dry cerrado savanna type biome even during relatively dry phases in the caracarana region where the savanna type vegetation biomass is lower and thus generates less charcoal particles than forest ecosystems

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

absence of last glacial maximum records in lowland tropical forests abstract environmental conditions of the lowland tropical forests during the last glacial maximum lgm between ca 20 000 and 18 00014c yr b p are reevaluated in terms of dating control and lithology analyzed in seven pollen records from south america the reevaluation shows that probably in none of the published records are lgm sediments present or abundant this conclusion is based on the occurrence of abrupt lithologic changes coupled with changes in sedimentation rate interpolated from radiocarbon dates these findings suggest that the lgm was represented probably by a hiatus of several thousand years indicative of drier climates than before or after

variations of the amazonian rainforest environment a sedimentological record covering 30 000 years abstract two cores css 2 and css 10 were collected from two lakes situated in the carajas area 6 35 s 49 30 w that show different stages of silting up the results of geochemical and petrographic organic analyses backed up by 14 c radiometric dating lead to a reconstruction of the variations in the hydrological regime of these lakes over the last 30 000 years evidence is revealed for a period of drying up characterized by a sedimentary hiatus between 22 000 and 13 000 14 c years bp followed by a period of refilling between 13 000 and 11 000 14 c years bp 15 400 12 900 cal years bp the latter interval is primarily characterized by a mainly clastic sedimentary input rich in quartz kaolinite and iron hydroxides leading on to the authigenic formation of siderite this type of sedimentation implies the erosion of the small catchment basin during periods of transition between arid and humid climatic regimes from 8000 to 4000 14 c years bp 8900 4500 cal years bp as well as from 2700 to 1500 14 c years bp 2780 1360 cal years bp special hydrological conditions prevailed they are characterized in the sedimentary record by the presence of micro laminae containing wood charcoal interlayered with sponge spicule material this interlamination reflects the rapid alternation of dry and humid periods a comparison with the results obtained by pollen analysis shows that sedimentological indicators once filtered for strictly local effects can provide a full record of the regional scale variations in the environment

forest stability during the early and late holocene in the igapo floodplains of the rio negro northwestern brazil located at the northwestern part of the amazon basin rio negro is the largest black water river in the world and is one of the poorest studied regions of the amazon lowlands in the middle upper part of the rio negro were retrieved sediment cores form lake acarabixi which were analyzed using pollen spores charcoal and geochemistry the aim of this study was to detect the influences from humans and river dynamics on the vegetation history in the region two main periods of vegetation and river dynamics were detected from 10 840 to 8240 cal yr bp the river had a direct influence into the lake the lake had a regional input of charcoal particles which reflected the effect of the dry holocene period in the basin furthermore highland taxa such as hedyosmum and myrsine were found at that time along with igapo forest species that are characteristic to tolerate extended flooding like eschweilera macrolobium myrtaceae swartzia and astrocaryum during the late holocene 1600 to 650 cal yr bp more lacustrine phases were observed there were no drastic changes in vegetation but the presence of pioneer species like vismia and cecropia along with the signal of fires which pointed to human disturbances

late pleistocene glacial forest elements of brazilian amazonia abstract pollen data recovered from the sedimentary record of western amazonia is still inadequate to fully address climate changes over the last glaciation in this region the present work focuses vegetation dynamics from an area of southwestern amazonia during the past 42 000 cal yr b p based on the integration of previous and new pollen data sediment cores were sampled from two sites covered by tropical rainforest in fluvial terraces of the madeira river a major southern amazonian tributary the results indicated a significant proportion of cold adapted andean tree species represented by alnus 0 20 hedyosmum 1 15 podocarpus 0 5 illex 1 11 and weinmannia 0 1 at least between 43 163 42 018 cal yr b p and 10 394 10 240 cal yr b p during the holocene only pollen representative of herbs and modern amazonian vegetation persisted the new pollen record confirmed previous documented results that vegetation communities presently restricted to andean areas at altitudes higher than 2000 3000 m occupied this region of the amazonian lowlands close to the onset and probably also during the last glacial maximum

seismic imaging of source region in the 1976 ms 7 8 tangshan earthquake sequence and its implications for the seismogenesis of intraplate earthquakes

hydroclimate influences on modern pollen rain of upland southeastern amazonia for accurate interpretations of palaeopalynological data it is important to understand the influence of the current vegetation composition and characteristics on the formation of the palynological

the hill of six lakes revisited new data and re evaluation of a key pleistocene amazon site the new analyses of a sedimentary record of lake pata in the hill of six lakes in nw amazon and its correlation with other quaternary proxy records in the region provide new insights regarding the vegetation and climate of the lowland forest during the last glacial despite what has been reported previously in the literature the sedimentary and pollen records are not continuous the hill remained forested however clear signals of structural change are seen in the record which indicate that the area experienced a significantly drier climate during the last glacial maximum lgm the herbs and taxa that are known to be more dominant in seasonally dry forests were all more abundant during the glacial part of the record and the cool adapted elements were mixed with warm lowland elements which indicates a temperature depression a comparison of the palaeoecological data with other regional geoenvironmental records of the upper negro river basin and other areas of the amazon provides additional support for a cooler and more seasonal environment during the middle pleniglacial which then became drier during the lgm a wet lgm is strongly refuted therefore the palaeoclimatic and ecological models that used the previous proxy data from six lakes to sustain wet conditions and a continuous forest record during the lgm to reconstruct the palaeoenvironmental conditions in the amazon should be reviewed

amazon plant diversity and climate through the cenozoic abstract the modern amazon lowlands date from the mid cenozoic when the andean orogeny defined an eastward or northward draining basin of immense size as the andean orogeny continued throughout the neogene and quaternary periods this system approached a steady state with andean flanks eroding to fill subsiding basins along the andean foreland followed by secondary erosion of sediments and transport in rivers across 3000 km to the atlantic ocean pollen data suggest that tropical forests were established on these lowlands at least by the end of the miocene with many modern amazon genera already present such antiquity is consistent with hypotheses that explain high diversity of modern amazon forests as the result of prolonged existence and minimal disturbance or extinction these museum models for amazon diversity came to be challenged by the paradigm of ice age aridity with its claim that the forests of the amazon lowlands were fragmented with each glacial advance into more or less tiny isolates called refugia separated by grass dominated communities like savanna for the latest pleistocene the arid amazon with forest refugia model can be tested by pollen analysis of lake sediments interpretation of amazon pollen data requires recognition both of the vast diversity of pollen types and an understanding of the dispersal of zoophilous pollen in the amazon amazon pollen analysis has been codified into a pollen atlas and this work used to review the available pollen data within the huge expanse of the amazon basin only three lacustrine records long enough and continuous enough to test this model have been published those from lake pata the carajas plateau and the amazon fan the review suggests community stability from all three data sets with no evidence for forest fragmentation in the amazon lowlands the principal response of amazon forest communities to climatic change is in the relative abundance of the more sensitive taxa particularly in response to interglacial warming a summary of geomorphological phenomena finds no evidence for glacial age aridity other than marginal reductions in precipitation within forest cover the amazon historical record is incomplete in time as well as in space there being essentially no paleobotanical data for much of the cenozoic particularly for the early pleistocene the data so far available however suggest stability and continuity of the forest cover over the vast amazon lowlands throughout the cenozoic as the circumstance in which the high amazon plant diversity evolved

aeolian activity in northern amazonia optical dating of late pleistocene and holocene palaeodunes palaeodunes were examined on the eastern margin of the rio branco rupununi savanna northeast amazonia optical dating suggests that the onset of aeolian activity was between 17 000 and 15 000 yr ago just after the last glacial maximum both the palaeodune axes and modern dominant wind directions have northeast to east northeast directions implying no significant shift in atmospheric circulation patterns over northeast amazonia during the late pleistocene to holocene major regional climate change events such as the younger dryas do not appear to have had any effect on the rates of aeolian deposition at the study site aeolian activity appears to have continued to the present day showing a remarkably constant deposition rate of around 0 13 m kyr 1 initially increasing smoothly to the present until more palaeodunes in northern amazonia are dated it is impossible to determine if this record of gradual aeolian deposition is a reliable regional palaeoclimate indicator rather than being the result of local bioclimatic and geomorphological effects copyright 2004 john wiley sons ltd

opacity of the martian atmosphere from the mars orbiter laser altimeter mola observations in this paper we present an analysis of the mars orbiter laser altimeter mola reflectivity measurements from the mars global surveyor mgs spacecraft during period of solar longitudes ls 198 212 reflectivity can be interpreted as a product of the surface geometrical albedo and the two way atmospheric transmission relative surface albedos were obtained from the red filter of viking color mars digital mosaics mdim to derive opacities at the mola wavelength 1 064 m opacity measurements are interpreted in terms of atmospheric aerosol loading in the canyons we also observed sudden increase of opacity in the northern regions 65 n which we interpret as condensing ice clouds

a major step in the continental moho and its geodynamic consequences the taranaki ruapehu line new zealand summary receiver function analysis reveals a 7 km step like change in crustal thickness across the taranaki ruapehu line tr line of north island new zealand the tr line runs east west between the active andesite volcanoes of mt taranaki and ruapehu and marks the southernmost extent of subduction zone volcanism in new zealand north of the tr line receiver functions show a strong and sharp p to s ps conversion at 25 1 5 km depth which is interpreted as a shallow moho at the tr line the moho ps conversion deepens across a step to 32 km depth and weakens further south the moho deepens to 35 km most of the 7 km step change in crustal thickness occurs over a lateral distance of 8 km yet there is little surface or topographic manifestation of the line given fresnel zone considerations the dip of the moho offset could vary between 90 and 45 gravity seismic attenuation and electrical data all show that the tr line is not only a step in crustal thickness but also a profound lithospheric boundary as mantle properties such as attenuation qp 1 implied density and electrical resistivity change abruptly across the line an east west oriented cluster of earthquakes with hypocentral depths of 20 40 km is centred on the moho step we propose that the moho step the earthquakes and the rapid change in mantle properties across the tr boundary are causally related processes that could be responsible for the phenomena described here include the rapid removal of mantle lithosphere and lower crust to the north of the tr line

foundering of lower island arc crust as an explanation for the origin of the continental moho a long standing theory for the genesis of continental crust is that it is formed in subduction zones however the observed seismic properties of lower crust and upper mantle in oceanic island arcs differ significantly from those in the continental crust accordingly significant modifications of lower arc crust must occur if continental crust is indeed formed from island arcs here we investigate how the seismic characteristics of arc crust are transformed into those of the continental crust by calculating the density and seismic structure of two exposed sections of island arc kohistan and talkeetna the kohistan crustal section is negatively buoyant with respect to the underlying depleted upper mantle at depths exceeding 40 kilometres and is characterized by a steady increase in seismic velocity similar to that observed in active arcs in contrast the lower talkeetna crust is density sorted preserving only relicts about ten to a hundred metres thick of rock with density exceeding that of the underlying mantle specifically the foundering of the lower talkeetna crust resulted in the replacement of dense mafic and ultramafic cumulates by residual upper mantle producing a sharp seismic discontinuity at depths of around 38 to 42 kilometres characteristic of the continental mohorovicic discontinuity the moho dynamic calculations indicate that foundering is an episodic process that occurs in most arcs with a periodicity of half a million to five million years moreover because foundering will continue after arc magmatism ceases this process ultimately results in the formation of the continental moho

three dimensional attenuation structure of the hikurangi subduction zone in the central north island new zealand summary we use waveform data from 2708 earthquakes recorded by dense seismograph deployments in the central north island of new zealand to image the 3 d attenuation structure of the hikurangi subduction zone down to ca 300 km depth attenuation images are obtained by determining the quality factor of p waves qp using a t inversion with a previously determined 3 d seismic velocity model we have included limited frequency dependence for qp with qp being frequency independent above 10 hz and having a frequency dependence of f 10 0 5 below 10 hz the qp images provide further constraint on the large scale features associated with subduction and magmatism beneath the central north island and serve to refine interpretations of crust and upper mantle structure from vp and vp vs the subducted plate is the most prominent feature in the qp images its high qp 900 1200 is consistent with the ca 120 myr old slab being relatively cold qp is better at resolving the slab than vp the coincidence of a strong gradient in qp with the upper plane of the dipping seismic zone indicates that even below 75 km depth the upper envelope of seismicity provides a good estimate of the location of the plate interface the mantle wedge is generally imaged as a relatively low qp 400 feature below 50 km depth however there are significant changes evident in the wedge along the strike of the subduction zone the most pronounced low qp in the mantle wedge occurs from ca 50 to 85 km depth beneath the productive rhyolite dominated central segment of the taupo volcanic zone suggesting a close link between volcanism and low qp in the shallow mantle wedge there is a strong correlation between low qp l owvp and high vp vs in this part of the mantle wedge suggesting that high temperature is the controlling influence on qp there within 30 km of the surface of the slab there is a large change in qp but only a modest change in vp our results are consistent with a fluid rich viscous blanket being entrained with the motion of the subducted slab down to ca 140 km depth where hydrous melting of mantle peridotite initiates trenchward of the volcanic front a strong horizontal gradient to higher qp is imaged in the mantle wedge we interpret this as a stagnant mantle nose not involved in the corner flow of the adjacent mantle the distribution of qp within both the slab and the overlying plate provides support for dehydration embrittlement being important in promoting seismicity in the mantle of the slab where qp is high at shallow depth in the forearc it is usually underlain by low qp near the plate interface this suggests that competent terranes in the forearc may act as aquicludes increasing fluid content in the rocks below and hence might control the distribution of coupling at the plate interface

hydrology and climatology at laguna la gaiba lowland bolivia complex responses to climatic forcings over the last 25 000 years diatom geochemical and isotopic data provide a record of environmental change in laguna la gaiba lowland bolivia 17 45 s 57 35 w over the last ca 25 000 years high resolution diatom analysis around the last glacial interglacial transition provides new insights into this period of change the full and late glacial lake was generally quite shallow but with evidence of periodic flooding at about 13 100 cal a bp just before the start of the younger dryas chronozone the diatoms indicate shallower water conditions but there is a marked change at about 12 200 cal a bp indicating the onset of a period of high variability with rising water levels punctuated by periodic drying from ca 11 800 to 10 000 cal a bp stable deeper water conditions persisted there is evidence for drying in the early to middle holocene but not as pronounced as that reported from elsewhere in the southern hemisphere tropics of south america this was followed by the onset of wetter conditions in the late holocene consistent with insolation forcing conditions very similar to present were established about 2100 cal a bp a complex response to both insolation forcing and millennial scale events originating in the north atlantic is noted

crust and upper mantle structure of wanganui basin and southern hikurangi margin north island new zealand as revealed by active source seismic data

late quaternary environmental changes and the alluvial record in humid tropical environments abstract ice core and ocean drilling records now confirm the evidence from lakes rivers and mires concerning temperature depression in the humid tropics during the lgm by at least 5 c on land and this accords with the recent ccm1 model output there is less agreement about fluctuation in precipitation amount intensity and seasonality during the late quaternary pollen records remain indeterminate because of the absence of sites in lowland rainforest locations and a widespread hiatus in sedimentation spanning the lgm alluvial sediments have often been ignored by specialists from other disciplines but they also provide a valuable source of palaeoenvironmental information that reflects catchment wide conditions current evidence shows that prolonged sedimentation took place in stage 3 at sites in s america and se asia and this was followed by incision for up to 9 ka spanning the lgm streams either produced little sediment or were characterised by braided channels and sometimes fan deposits until the transition towards an interglacial mode took place after 12 700 bp 14 c in many places this transition was marked by large floods incision and deposition of coarse sediment but as the holocene climates stabilised and fluctuations of runoff reduced in amplitude so meandering rivers developed floodplains with thick overbank silts and clays these changes have regional and intercontinental patterns not everywhere identical in bracketing dates but demonstrating major changes to the fluvial systems as a result of profound changes to the hydrology of forested tropics during the last 60 ka

origin transport and deposition of leaf wax biomarkers in the amazon basin and the adjacent atlantic paleoenvironmental studies based on terrigenous biomarker proxies from sediment cores collected close to the mouth of large river systems rely on a proper understanding of the processes controlling origin transport and deposition of biomarkers here we contribute to the understanding of these processes by analyzing long chain n alkanes from the amazon river system we use the dd composition of long chain n alkanes from river bed sediments from the amazon river and its major tributaries as well as marine core top samples collected off northeastern south america as tracers for different source areas the d13c composition of the same compounds is used to differentiate between long chain n alkanes from modern forest vegetation and petrogenic organic matter our d13c results show depleted d13c values 33 to 36 in most samples indicating a modern forest source for most of the samples enriched values 31 to 33 are only found in a few samples poor in organic carbon indicating minor contributions from a fossil petrogenic source long chain n alkane dd analyses show more depleted values for the western tributaries the madeira and solimoes rivers 152 to 168 while n alkanes from the lowland tributaries the negro xingu and tocantins rivers 142 to 154 yield more enriched values the n alkane dd values thus reflect the mean annual isotopic composition of precipitation which is most deuterium depleted in the western amazon basin and more enriched in the eastern sector of the basin samples from the amazon estuary show a mixed long chain n alkane dd signal from both eastern lowland and western tributaries marine core top samples underlying the amazon freshwater plume yield dd values similar to those from the amazon estuary while core top samples from outside the plume showed more enriched values although the variability in the river bed data precludes quantitative assessment of relative contributions our results indicate that long chain n alkanes from the amazon estuary and plume represent an integrated signal of different regions of the onshore basin our results also imply that n alkanes are not extensively remineralized during transport and that the signal at the amazon estuary and plume includes refractory compounds derived from the western sector of the basin these findings will aid in the interpretation of plant wax based records of marine sediment cores collected from the adjacent ocean

modern pollen rain as a background for palaeoenvironmental studies in the serra dos carajas southeastern amazonia several alternative interpretations of the pollen records of lake sediments in the amazonia may arise due to a lack of understanding of the modern environments mainly related to pollen vegetation

the role of tectonics and climate in the late quaternary evolution of a northern amazonian river abstract the amazon basin has most of the largest rivers of the world however works focusing the geological evolution of the trunk river or its tributaries have been only partly approached the branco river constitutes one of the main northern amazonian tributaries a previous work proposed that before flowing southward into the negro amazon rivers the branco river had a southwest to northeast course into the caribbean sea the present work aimed to establish if the proposed change in the course of this river is supported by morphological and sedimentological data other goals were to discuss the factors influencing river development and establish its evolution over time within the chronological framework provided by radiocarbon and optically stimulated luminescence dating the work considered the entire course of the branco river downstream of the precambrian guiana shield where the river presumably did not exist in ancient times the river valley is incised into fluvial sedimentary units displaying ages between 100 and 250 ky old which record active and abandoned channels crevasse splay levees and point bars the sedimentary deposits in the valley include two alluvial plain units as old as 18 7 ky and which intersects a late pleistocene residual megafan these characteristics suggest that a long segment of the branco river was established only a few thousand years ago together with several structural anomalies these data are consistent with a mega capture at the middle reach of this river due to tectonic reactivation in the late pleistocene this integrated approach can be applied to other amazonian tributaries to unravel how and when the amazonian drainage basin became established

age of depositional and weathering events in central amazonia abstract in the last three decades several studies have been devoted to understanding the role of late pleistocene holocene climate changes in the amazonia lowlands environment however most of these studies used data obtained from sedimentary deposits lakes swamps and colluvium located away from the central plain or on the edges of the amazonia region this article integrates optically stimulated luminescence and accelerated mass spectrometry 14 c ages with sedimentological and geomorphological data obtained during this study or compiled from the literature for fluvial and lacustrine deposits of the central alluvial plain of the solimoes amazon river the age data allow us to present a chronological framework for the late pleistocene holocene deposits and conclude that i the dryness of the lgm in central amazonia lowlands is recorded by the formation of fluvial terraces and their weathering to pedogenic hematite between 25 3 ka and 17 7 ka ii floodplain deposition was contemporaneous with terrace weathering and occurred in a context of decreased water volume in fluvial channels lowering of river base level and sea level and isostatic rebound of the continent and iii lateral and mid channel fluvial bars in the solimoes amazon river have a minimum age of 11 5 1 5 ka and their deposition responded to increased precipitation at the beginning of the holocene

the response of vegetation on the andean flank in western amazonia to pleistocene climate change a reconstruction of past environmental change from ecuador reveals the response of lower montane forest on the andean flank in western amazonia to glacial interglacial global climate change radiometric dating of volcanic ash indicates that deposition occurred 324 000 to 193 000 years ago during parts of marine isotope stages 9 7 and 6 fossil pollen and wood preserved within organic sediments suggest that the composition of the forest altered radically in response to glacial interglacial climate change the presence of podocarpus macrofossils 1000 meters below the lower limit of their modern distribution indicates a relative cooling of at least 5 c during glacials and persistence of wet conditions interglacial deposits contain thermophilic palms suggesting warm and wet climates hence global temperature change can radically alter vegetation communities and biodiversity in this region

new seismological constraints on growth of continental crust in the izu bonin intra oceanic arc the process by which continental crust has formed is not well understood though such crust mostly forms at convergent plate margins today it is thus imperative to study modern intra oceanic arcs such as those common in the western pacific ocean new seismic studies along the representative izu bonin intra oceanic arc provide unique along strike images of arc crust and uppermost mantle to complement earlier cross arc lithospheric profiles these reveal two scales 1000 10 km scale of variations one at the scale of the izu versus bonin thick versus thin arc crust the other at the intervolcano 50 km scale these images show that 1 the bulk composition of the izu bonin arc crust is more mafic than typical continental crust 2 the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes 3 the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments and 4 a process to return lower crustal components to the mantle such as delamination is required for an arc crust to evolve into continental crust continued thickening of the izu bonin crust accompanied by delamination of lowermost crust can yield velocity structure of typical continental crust

gravitational instability of mantle lithosphere and core complexes for a wide range of viscosity structures convergent and downward flow of the mantle lithosphere during the growth of gravitational instability induces not only thickening of overlying crust but also concurrent horizontal extension in the upper crust such extension if it occurred in the earth would include normal faulting of the upper crust above a region of horizontal shortening in the lower crust and uppermost mantle convergent flow in the lower crust would also create shear stress on horizontal planes and localized upward flow of the lower crust these features extension of upper crust and exhumation of strained lower crust characterize metamorphic core complexes exposed in regions of normal to thick continental crust thus convergent flow and downwelling mantle lithosphere might contribute to the development of core complexes at least in some settings if horizontal shortening and crustal thickening at depth do occur simultaneously with normal faulting at the surface of the earth today evidence of this process does not seem obvious but perhaps it has occurred concurrently with widespread regional crustal extension in places like the basin and range province tibet the pamir or the aegean if such mantle flow does participate in the development of core complexes a weak lower crust might not be a prerequisite for their formation

pliocene exhumation and landscape evolution of central north island new zealand the role of the upper mantle 1 mudstone porosity analyses show that over 2 km of exhumation has occurred in central north island new zealand since 5 ma this combined with a coeval surface uplift of 500 m indicates a maximum 2 5 km of rock uplift in central north island the magnitude and wavelength 400 km of the rock uplift is similar to some topographic doming observed in continental areas associated with mantle upwelling yet central north island is recognized as a region of back arc extension marine terraces 100 150 km from the center of maximum exhumation show uplift rates of 0 2 0 3 mm yr 1 consistent with about 1 km of exhumation if averaged over 4 5 myr which compares well with mudstone porosity analyses seismic attenuation and slow pn velocities indicate that a thermal anomaly in the upper mantle is the source of this rock uplift the rock uplift may be explained by the replacement of a 100 km thick mantle lid by asthenosphere with a density 66 kg m 3 less than the lid about half this density contrast can be explained by thermal expansion and the rest is ascribed to partial melts in the upper mantle southward migration for the locus of maximum rock uplift is consistent with the zone of maximum incision erosion being 50 km south of the southernmost volcano such a migration is in keeping with other geological markers of volcanic and thermal phenomena and further strengthens the link of landscape evolution at active continental margins with processes in the mantle

patterns of anabranching channels the ultimate end member adjustment of mega rivers abstract large fluvial systems adjust to a combination of controls to form distinctive channels which represent a dominant factor in the evolution of floodplain geomorphology and sedimentology fluvial geomorphology has commonly classified river channels into meandering straight and braiding patterns which are seen to represent a continuum of channel geometry anabranching patterns rivers with multiple channels however are characteristic of many rivers the identification of a combination of variables that discriminates specific channel patterns has been a significant focus of research in fluvial geomorphology the development of this body of knowledge however has been established from medium and small rivers and laboratory flume studies very few of these research ideas developed from analysis of large fluvial systems this paper assesses the pattern of channel adjustment of large fluvial systems by employing hydraulic geometry discharge w d slope grain size stream power specific stream power and froude number q mean 1000 m 3 s the study demonstrates that methods currently used to discriminate channel patterns are not useful when applied to very large rivers further with the exception of the lower mississippi alluvial rivers with mean annual discharges greater than 17 000 m 3 s here classified as mega rivers do not generate single thread meandering or typical braided patterns these mega rivers develop anabranching patterns

late quaternary vegetation and climate change in the amazon basin based on a 50 000 year pollen record from the amazon fan odp site 932 hemipelagic sediments from the amazon deep sea fan odp site 932 5 12 7 n 47 1 8 w and continental shelf provide a 50 000 yr long pollen record of amazon basin vegetation the age model for hole 932a is constrained by eight magnetic remanence intensity features one paleomagnetic excursion and three ams14c dates alchornea melastomataceae myrtaceae and moraceae urticaceae are dominant taxa in the pollen record between 40 200 and 19 800 cal yr b p andean taxa such aspodocarpusandhedyosmum increase in abundance between 19 800 and 11 000 cal yr b p and prior to 40 200 cal yr b p the holocene pollen assemblage derived from amazon river and continental shelf sediments is dominated by secondary growth taxa such ascecropia climatic factors influencing the development of glacial and interglacial tropical vegetation are considered by comparing marine with terrestrial records of vegetation change this comparison shows that the amazon basin forests were not extensively replaced by savanna vegetation during the glacial period contradicting the refugia hypothesis

lithospheric gravitational instability beneath the southeast carpathians abstract the southeast corner of the carpathians known as the vrancea region is characterised by a cluster of strong seismicity to depths of about 200 km the peculiar features of this seismicity make it a region of high geophysical interest in this study we calculate the seismic strain rate tensors for the period 1967 2007 and describe the variation of strain rate with depth the observed results are compared with strain rates predicted by numerical experiments we explore a new dynamical model for this region based on the idea of viscous flow of the lithospheric mantle permitting the development of local continental mantle downwelling beneath vrancea due to a rayleigh taylor instability that has developed since the cessation of subduction at 11 ma the model simulations use a lagrangean frame 3d finite element algorithm solving the equations of conservation of mass and momentum for a spatially varying viscous creeping flow the finite deformation calculations of the gravitational instability of the continental lithosphere demonstrate that the rayleigh taylor mechanism can explain the present distribution of deformation within the downwelling lithosphere both in terms of stress localisation and amplitude of strain rates the spatial extent of the high stress zone that corresponds to the seismically active zone is realistically represented when we assume that viscosity decreases by at least an order of magnitude across the lithosphere the mantle downwelling is balanced by lithospheric thinning in an adjacent area which would correspond to the transylvanian basin crustal thickening is predicted above the downwelling structure and thinning beneath the basin

the mars observer laser altimeter investigation the primary objective of the mars observer laser altimeter mola investigation is to determine globally the topography of mars at a level suitable for addressing problems in geology and geophysics secondary objectives are to characterize the 1064 nm wavelength surface reflectivity of mars to contribute to analyses of global surface mineralogy and seasonal albedo changes to assist in addressing problems in atmospheric circulation and to provide geodetic control and topographic context for the assessment of possible future mars landing sites the principal components of mola are a diode pumped neodymium doped yttrium aluminum garnet laser transmitter that emits 1064 nm wavelength laser pulses a 0 5 m diameter telescope a silicon avalanche photodiode detector and a time interval unit with 10 ns resolution mola will provide measurements of the topography of mars within approximately 160 m footprints and a center to center along track foot print spacing of 300 m along the mars observer subspacecraft ground track the elevation measurements will be quantized with 1 5 m vertical resolution before correction for orbit and pointing induced errors mola profiles will be assembled into a global 0 2 deg x 0 2 deg grid that will be referenced to mars center of mass with an absolute accuracy of approximately 30 m other data products will include a global grid of topographic gradients corrected individual profiles and a global 0 2 deg x 0 2 deg grid of 1064 nm surface reflectivity

environmental and vegetation changes in southeastern amazonia during the late pleistocene and holocene abstract new insights about climate conditions and their effect on vegetation and lakes of southeastern amazonia were achieved over the last 50 cal ka using recent data obtained from a filled lake located in the serra sul de carajas our analysis was done in a sediment core r2 comprising of faciological palynological isotopic and radio carbon measurement supported by bayesian age depth modeling as well species distribution modeling wet and cool climate conditions are proposed for the mid pleniglacial which is favorable to intense weathering of the lake s catchment area and the expansion of high altitude taxa cool adapted into forest formations during this period from the late pleniglacial to the last glacial maximum a change from relatively wet to dry climate possibly converted the studied lake from oxidizing to reducing conditions which leads to the formation of diagenetic siderites at the end of this time interval warming during the pleistocene to holocene transition caused the disappearance of cool adapted taxa in the study site concomitantly increasing precipitation allowed the expansion of forest and high lake levels a drier early mid holocene produced extensive open vegetation and desiccated lakes current climate and environmental conditions were likely attained in the region from the mid to late holocene

thinning of continental backarc lithosphere by flow induced gravitational instability abstract many continental backarcs have thin 60 km lithosphere for 100s of km behind the volcanic arc even where there has been no significant extension one mechanism to produce thin backarc lithosphere is through gravitational thinning of normal thickness lithosphere by subduction related mantle flow the stability of backarc mantle lithosphere is examined using thermal mechanical models of subduction of an oceanic plate beneath continental lithosphere with an initial thickness of 120 km and a thermal structure similar to average phanerozoic continental lithosphere subduction induced mantle flow shears the base of the backarc lithosphere producing lateral density perturbations owing to the non newtonian lithosphere rheology shearing also reduces the effective viscosity of the lowermost lithosphere enabling the density perturbations to become gravitationally unstable the perturbations develop into rapidly growing downwellings which result in removal of lower backarc lithosphere on timescales of 5 10 ma conductive heating and shearing of the remaining lithosphere results in a second more muted phase of gravitational instability and thinning lower lithosphere instability is enhanced by higher subduction rates weaker intrinsic rheology higher compositional density and hotter initial thermal structure the numerical model results are in good agreement with a buoyancy stability analysis which includes the vertical gradients in temperature and strain rate through the lithosphere as both rheology and density depend on lithosphere composition significant thinning may be restricted to continental mantle lithosphere that is fertile and contains a small amount of water to produce a final thickness comparable to that observed without lithospheric contraction or extension it is necessary to have a weak lithosphere rheology an initial thermal structure hotter than average phanerozoic continental geotherms or a combination of the two the region of thin lithosphere at the northern cascadia backarc coincides with terranes that were accreted to the north american craton the known fertile composition of the backarc lithosphere may allow it to be thinned while the drier more refractory craton lithosphere is resistant to thinning

neogene and quaternary development of the neotropical rain forest the forest refugia hypothesis and a literature overview abstract the upheaval of the northern andes in miocene and pliocene time changed the drainage system in northern south america significantly and caused the present day rain forest areas of choco and the lower magdalena valley became separated from amazonas plant diversity may have reached the highest level in the miocene or pliocene and excessive present day phytodiversity may be regarded as a legacy of the tertiary rather than an evolutionary product of the quaternary in the quaternary strong temperature oscillations related to the series of ice ages were superposed on the late tertiary forest dynamics which included river displacement and latitudinal migrations of the equatorial rain belt caloric equator with the rhythm of the precession cycle of orbital climate forcing the hypothesis that claims a permanent rain forest cover all over the amazon basin during the last glacial is in contrast with the forest refugia hypothesis which accepts replacement of rain forest by savanna or savanna forest during dry climatic intervals both scenarios have been evidenced by pollen records in this paper it is suggested that both hypotheses are not necessarily conflicting and apparently did occur in different parts of the amazon basin and in different periods depending on the climatological constraints a compilation of the most important literature concerning the vegetational climatic and environmental history of the rain forest areas of amazonas and choco and surrounding dry ecosystems has been included

amazonian paleodunes provide evidence for drier climate phases during the late pleistocene holocene abstract recent studies on relict eolian dunes in the rio negro basin northern amazon brazil 00 35 n 63 14 w indicate a drier climate regime during the late pleistocene holocene transition that is different from the present humid climate the eolian sands form long chains of linear dunes bordering the rio negro river and some tributaries here we present thermoluminescence tl ages spanning the period 32 000 8000 yr b p the final dune stabilization took place after 8 000 yr b p and now the bases of the dunes are fixed by vegetation clustering of the tl dates suggests that the dry climate in the amazon basin occurred in distinct episodes and argues against current opinions that drastic ecological changes did not affect in the amazon during the last global glaciation

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

biogeochemical indicators of environmental changes from 50 ka to 10 ka in a humid region of the brazilian amazon abstract we present a geochemical record of a 113 6 cm sediment core lpt v from lagoa da pata which is located in the forested upper rio negro basin the record reveals significant changes in the environmental history of amazonia during the late quaternary the results of biogeochemical analyses revealed three hydrological and climatic regimes from 50 000 to 10 000 cal yr bp the first phase between 50 000 and 26 300 cal yr bp was characterized by a relatively wet climate as suggested by relatively high total organic carbon toc and chlorophyll derivate concentrations indicating high productivity linked to a high lake level a decrease of the toc and chlorophyll derivate accumulation rates between 43 100 cal yr bp and 26 300 cal yr bp marks a decrease in the productivity linked to a reduced lake level indicating a decrease in moisture at the end of this phase the second phase between 26 300 and 15 300 cal yr bp was characterized by a decrease in productivity reaching a minimum at 21 950 cal yr bp as indicated by a minima in sedimentary chlorophyll and toc accumulation rates values of 13 c increased by 5 in relation to the preceding phase indicating an influence of the c 4 organic matter high iron concentrations and accumulation rates related to intense erosion of the lateritic crust in the watershed were observed all of the observations indicate a dry phase during this period a third phase from approximately 15 300 to 10 000 cal yr bp was characterized by an increase in lacustrine productivity as shown by an increased in toc and chlorophyll derivate concentrations and accumulation rates these increases likely correspond to a lake level rise due to a wetter climate

paleoenvironmental dynamics in south amazonia brazil during the last 35 000 years inferred from pollen and geochemical records of lago do saci abstract paleoenvironmental changes for the last 35 000 years were reconstructed from palynological sedimentological and organic geochemical evidence from a well dated sediment core from lago do saci south amazonia dry climatic conditions occurred between 35 000 and 18 200 cal yr bp as recorded by high frequencies of open savanna taxa and low lake level indicated by sagittaria and low total organic carbon content cold temperatures are indicated by the presence of podocarpus and ilex a sedimentation hiatus observed between 18 200 and 9200 cal yr bp was likely related to dry conditions the beginning of the holocene was marked by rainforest expansion and an increase in carbon content that represented high lake levels and warmer and wetter climate conditions between 7500 and 5000 cal yr bp the expansion of open savanna seasonal forest elements and abundant black carbon suggests a dry phase after 5000 cal yr bp rainforest expansion and higher lake levels indicate a return to wetter conditions a reduction of flooding taxa celtis and mauritia between 1800 and 1300 cal yr bp high lake level conditions and maintenance of a forest physiognomy suggests a decrease of regional precipitation and subsequent reduction of the flooded areas in a still humid climate regime

topography of the northern hemisphere of mars from the mars orbiter laser altimeter the first 18 tracks of laser altimeter data across the northern hemisphere of mars from the mars global surveyor spacecraft show that the planet at latitudes north of 50 is exceptionally flat slopes and surface roughness increase toward the equator the polar layered terrain appears to be a thick ice rich formation with a non equilibrium planform indicative of ablation near the periphery slope relations suggest that the northern tharsis province was uplifted in the past a profile across ares vallis channel suggests that the discharge through the channel was much greater than previously estimated the martian atmosphere shows significant 1 micrometer atmospheric opacities particularly in low lying areas such as valles marineris

mantle earthquakes crustal structure and gravitational instability beneath western north island new zealand a cluster of 30 52 km deep earthquakes and a 7 10 km step in the moho beneath western north island new zealand are both interpreted as manifestations of active delamination of the continental lower crust and mantle lithosphere these phenomena occur in the back arc region beneath the east west oriented taranaki ruapehu tr line which strikes at a high angle to the present day plate boundary through new zealand across the line there is an abrupt change in crustal and mantle lid thickness and in upper mantle seismic attenuation q p 1 showing that the mantle lithosphere has been highly thinned on the north side we show from a receiver function profile that nearly all of the deep earthquakes reside in the uppermost mantle on the northern side of the tr line a sum of earthquake moment tensors suggests strike slip motion either parallel or orthogonal to the tr line resulting in northwest southeast oriented horizontal extension active normal faults oriented northwest southeast and north south are seen at the surface on each side of the tr line but the surface is uplifting here at 0 4 mm yr this requires the mantle lithosphere to be thinning at a higher rate than the overlying crust consistent with a delamination process

evidence for late quaternary aeolian activity in the roraima guyana region abstract new evidence is provided for the aeolian origin of the upper unit of the boa vista formation found in the eastern part of the brazilian state of roraima and in neighboring guyana this unit has two geomorphologically sedimentologically distinct domains a sandy and a silty realm located to the east and southeast upwind of the silty realm the sandy realm includes very highly eroded inactive parabolic and linear dunes and sand sheets an extensive field of parabolic dune remnants was detected in an orbital radar image and in low altitude air photos of the interfluve of the cauame and uraricoeira rivers near the town of boa vista along the downwind periphery of the sandy realm is a thin mantle of silty sediments forming a flat plain and penetrating into intermontane valleys the eroded fossil dunes oriented 50 60 e and the downwind grain size selection suggest a late pleistocene dry period with trade wind direction the same as that of the modern dry season a more recent dry period probably holocene is suggested by the presence of many well preserved circular depressions in the upper unit of the boa vista formation interpreted as circular pans or blowout hollows