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

use of environmental magnetic measurements to validate the vertical extent of ice masses at the last glacial maximum analysis of soil samples from above and below trimlines representing the upper limit of glacial erosion at the last glacial maximum demonstrates that soils with prolonged weathering histories above such trimlines yield significantly different mineral magnetic signatures from soils below trimlines the nature of the contrast is conditioned by lithology basalt soils above the trimline yield significantly higher values of concentration dependent magnetic parameters arm irm3t soft irm and hard irm than those below the trimline due probably to transformation of non magnetic iron bearing minerals into magnetic forms conversely for sandstone soils most magnetic parameters yield significantly lower values for above trimline samples probably reflecting loss of ferrimagnetic minerals by dissolution and oxidation to aniferrimagnetic forms these significant contrasts represent a new approach to validating high level weathering limits as periglacial trimlines cut at the last glacial maximum copyright 2002 john wiley sons ltd

understanding amazonian fluvial rias based on a late pleistocene holocene analog fluvial rias are elongated lakes at tributary mouths that can reach dozens of kilometers in length constituting one of the most remarkable features in the amazonian landscape thus far definitive data which documents the genesis of fluvial rias have not been published the main goal of this work was to integrate morphological sedimentological and chronological information in order to characterize fluvial paleorias in the interfluve of the purus and madeira rivers and discuss the most likely hypothesis for their genesis these paleorias were first observed through remote sensing imagery as several elongated and interconnecting belts of open vegetation that are in sharp contact with the surrounding dense forest the belts are branched and form a dendritic pattern similar to many modern drainage networks the sedimentary record of these belts revealed the prevalence of sharp based sandstones and mudstones arranged into fining upward successions which are compatible with deposition within channels active channel and abandoned channel deposits were recognized these are topped by continuous mudstones related to rapid channel abandonment and formation of a low energy basin or ria environment radiocarbon dating of these deposits recorded only late pleistocene and holocene ages ranging from 21 547 22 285 cal yr bp to 5928 6124 cal yr bp this chronology for sediment deposition is not compatible with the hypothesis of amazonian rias being formed by fluvial erosion during the last glaciation maximum low sea level with sediment accumulation during the subsequent holocene transgression instead the studied paleorias record previous tributaries of the madeira river that became abandoned as the position of this river shifted southeastward and its interfluve tilted northward inverting the drainage systems therefore a neotectonic origin of some amazonian paleorias seems most likely this hypothesis should be considered in further investigations aiming at understanding the origin of numerous modern fluvial rias that typify the amazonian landscape copyright 2014 john wiley sons ltd

andean tectonics as a cause for changing drainage patterns in miocene northern south america newdatafromneogenestratainnorthernsouthamericasuggestthatmiocenetectonisminthenortheasternandeswasresponsible for the genesis of the amazon river and changes in the drainage patterns of other major rivers such as the magdalena and the orinoco here we present a new model for the paleogeographic evolution of northern south america during the miocene in the early miocene a large part of the drainage of northwest amazonia was directed northward along the paleo orinoco river system to a deltainlakemaracaibo upliftoftheeasterncordillerainthelate middle miocene caused thefirst development of the amazon river however no connection with the atlantic was established and the amazon fed the paleo orinoco river system which drained toward the caribbean substantial andean uplift in the late miocene resultedinmajorchangesinpaleogeography theorinocochangedits course the amazon established a connection to the atlantic causing the drowning of carbonate platforms and the amazon caribbeanconnectionwasclosed thusthedrainageandpaleogeography ofnorthernsouthamericainthemiocenewerestronglycontrolled by tectonic movements in the northeastern andes

late quaternary fluvial terrace evolution in the main southern amazonian tributary abstract late quaternary sedimentary deposits from amazonian lowlands are important geological archives for understanding the origin evolution and controls of the largest fluvial drainage basin on earth climate has been most often claimed as the main factor affecting river evolution in this region an increasing volume of publications have also highlighted tectonics as a relevant control this work investigates the late quaternary evolution of the madeira river the biggest southern tributary of the amazonas basin and a major waterway in south america with the goal of discussing the factor with highest potential of influence on its development in space and time the approach consisted of carrying out a detailed morphological sedimentological and chronological characterization of terrace deposits three terraces were recognized which record sandy pebbly channel muddy channel oxbow lake point bar floodplain as well as crevasse channel crevasse splay and levee deposits the topographically highest terrace t1 formed in the time interval before 43 500 and 31 696 32 913 cal yrs bp the intermediate terrace t2 between 25 338 26 056 and 14 129 14 967 cal yrs bp and the lowest terrace t3 between 12 881 13 245 and 3158 3367 cal yrs bp these terraces are the testimony of successive downcutting and sediment aggradation episodes of terrace erosion and deposition cannot be fully accommodated within the framework of fluctuating river base level tied to late pleistocene holocene global or regional climate fluctuations changes in tectonic subsidence rates might have played an important role in the equilibrium state of this particular fluvial system

evolution of the western amazon lowland relief impact of andean foreland dynamics the congruency in the depositional origin and age of the uppermost sedimentary strata forming non flooded rainforest ground terra firme in the western and central amazon lowlands is a much debated subject here we conclude from the study of remote sensing imagery that active andean foreland dynamics have played a major role in the evolution of the plio pleistocene fluvial landscape in the western amazon foreland dynamics have resulted in a terra firme composed of late tertiary alluvium and younger alluvial terraces and plains in peru thermoluminescence and 14c dating show local aggradation of this younger alluvium between 180 and 30 ka the documented high age heterogeneity of the terra firme has implications for considerations of the biogeography of the amazon forest

erosion of an active fault scarp leads to drainage capture in the amazon region brazil far from the continental margin drainage basins in central amazonia should be in topographic steady state but they are not abandoned remnant fluvial valleys up to hundreds of square kilometers in size are observed throughout amazonia and are evidence of significant landscape reorganization while major late miocene drainage shifts occurred due to initiation of the transcontinental amazon river local landscape change has remained active until today driven either by dynamic topography tectonism and or climatic fluctuations drainage captures in amazonia provide a natural experiment for assessing the geomorphic response of low slope basins to sudden capture related base level falls this paper evaluates the timing of geomorphic change by examining a drainage capture event across the baependi fault scarp involving the cuieiras and taruma mirim river basins northwest of the city of manaus in brazil a system of capture related knickpoints was generated by base level fall following drainage capture through numerical modeling of their initiation and propagation the capture event is inferred to have occurred between the middle and late pleistocene consistent with other studies of landscape change in surrounding areas in low slope settings like the amazon river basin base level fall can increase erosion rates by more than an order of magnitude and moderate to large river basins can respond to episodes of base level fall over timescales of tens to hundreds of thousands of years copyright 2013 john wiley sons ltd

the variation of heat generation density and seismic velocity with rock type in the continental lithosphere abstract the petrophysical properties radioactive heat generation compressional wave velocity and density are related to rock type in crystalline rocks the cation fraction in the rock forming minerals is applied to determine the relationships quantitatively heat generation exhibits considerable variation with rock type as well as with the geologic age of the rocks therefore the relation between heat generation and rock type has been split into a relation for phanerozoic rocks and another for precambrian rocks which exhibit significantly lower heat generation only 1 3 of that of phanerozoic rocks the interrelations between the petrophysical properties are applied to fennoscandia precambrian terrane and southern bavaria northern alpine foreland phanerozoic rocks to estimate the heat generation by radioactivity in the crust and the heat flow density from the upper mantle

late quaternary palaeoecology and palaeoclimatology from pollen records of the savannas of the llanos orientales in colombia abstract pollen analysis of sediment cores from laguna angel 4 28 n 70 34 w 200 m elevation and laguna sardinas 4 58 n 69 28 w 80 m a s l in the savannas of the llanos orientales of colombia are presented the pollen record of laguna angel includes 7 radiocarbon dates and shows the holocene vegetation and climatic history since 10 030 yr b p the record of laguna sardinas includes 7 radiocarbon dates and shows the late glacial and holocene history since 11 570 yr b p both pollen records show that the present day savanna vegetation dominated by grassland poaceae with only few woody savanna taxa e g curatella bryrsonima was present since the late glacial indicating dry climatic conditions with a marked dry season shrubs and trees of forest and or gallery forest e g mauritia mauritiella moraceae urticaceae melastomataceae alchornea and myrtaceae are well represented during the recorded period the proportions between forest and savanna were subject to change in the past but also the composition of the forest and savanna ecosystems changed laguna sardinas shows a greater abundance during the late glacial holocene transition than today for alchornea especially from 10 680 to 10 070 yr b p for this interval synchronous to the el abra stadial younger dryas equivalent in colombia more humid climatic conditions have been inferred which are primarily the result of higher moisture levels under colder conditions during the early and middle holocene from 9730 to 5260 yr b p in laguna angel and from 9390 to 6390 yr b p in laguna sardinas the maximum expansion of grassland occurred indicating very dry climatic conditions the following period from 5260 to 3890 yr b p in laguna angel and from 6390 to 3680 yr b p in laguna sardinas shows a decrease of savanna herbs and an increase of forest and gallery forests reflecting a wetter period probably with a shorter annual dry season in both lake records the period after ca 3500 yr b p is incomplete but is characterized by a high representation of the palm genera mauritiella in laguna angel and mauritia in laguna sardinas this marked change has been interpreted as evidence of strong a anthropogenic influence on the vegetation but also wetter climatic conditions than during the late glacial and early and middle holocene periods

marine incursions and the influence of andean tectonics on the miocene depositional history of northwestern amazonia results of a palynostratigraphic study abstract new palynological and sedimentological data permit the age of neogene sediments in northwestern amazonia to be established more precisely and indicate that major environmental changes occurred in the area during this time based on a study of borehole samples the age of the solimoes formation solimoes basin northwestern brazil is determined as miocene and five pollen zones are distinguished which are correlated with existing zonations for northern south america in addition 30 new sporomorphs are described which belong to the following 12 genera psilamonocolpites retimonocolpites retitricolpites retibrevitricolpites psilatriporites nov gen bombacacidites psilatricolporites retitricolporites rugutricolporites psilastephanoporites nov gen psilastephanocolporoties and heterocolpites a correlation is made between the brazilian wells and some of the studied outcrops in colombian and peruvian amazonia based on palynological marker species the presence of coastal elements such as mangroves zonocostites indicates that during the miocene this area was influenced by marginal marine conditions caused by several marine incursions these incursions may be related to global sea level fluctuations miocene marine phases also are known from sites elsewhere in northern south america which at present like northwestern amazonia are entirely ruled by continental conditions sediment composition shows that during the early miocene the guyana shield was the major source area of sediment input in the basins of northwestern amazonia in the interval from early to middle miocene the andes became the major sediment source area the change in provenance is related to the uplift of the eastern cordillera this event caused a major change in palaeoenvironment and palaeogeography in northwestern amazonia which was characterized by the reverse of a northwestward directed fluvial system into an eastward directed fluvial lacustrine system with an estuarine character

fluvial archive of peculiar avulsive fluvial patterns in the largest quaternary intracratonic basin of tropical south america the bananal basin central brazil abstract the bananal basin is the most important intracratonic quaternary sedimentary basin of south america and preserves a good record of the paleohydrological conditions of central brazil at the cerrado amazon ecotone this area is currently a large tropical plain seasonally flooded by rainfall and water table saturation that can be classified as a seasonal wetland this paper presents a geological and geomorphological analysis supported with geochronological dating aiming to reconstruct the pleistocene environmental history of the bananal basin the basin was filled by the sediments sourced by the araguaia mortes fluvial system the deposits belong to the araguaia formation which covers 106 000 km 2 of the bananal basin tl and osl chronologies of the araguaia formation show that sedimentation started at least in the middle pleistocene mis 7 but aggradation and avulsion processes were important in the lower middle pleniglacial between 70 5 ka and 34 0 4 6 ka mis 4 and 3 and in the upper pleniglacial between 24 5 3 1 and 17 2 2 3 ka mis 2 river avulsions were more intense in the lower bananal basin bananal island region remarkably we did not record ages between 17 ka and 10 ka although avulsion seems to have been more active in the past the abandonment by the araguaia of a channel belt now occupied by the underfit javaes river indicates that avulsion is still active the avulsion processes are mainly related to palaeohydrological changes although present day seismic activity is also recorded additional data is needed to generate a more detailed scenario for the older middle pleistocene deposits the data is remarkable however because it is the first middle pleistocene record described in a large alluvial tropical system in brazil the fluvial record is temporally in agreement with records from other large brazilian fluvial systems such as the amazon and parana basins the bananal plain however is unique because of its particular style of sedimentation and geomorphologic processes that generated a mega scale anabranching like complex pattern of paleochannels presently occupied by underfit streams

quaternary fluvial systems of tropics major issues and status of research abstract fluvial systems in tropical regions are not only the lifeline for modern civilizations there but also form an important continental archive for reconstructing quaternary palaeoclimates and palaeoenvironments tropical rivers drain through a variety of geological and geomorphological settings across the world and have been studied for their hydrology sediment transport characteristics facies models stratigraphic development and flood hazards this paper presents an overview of the research on tropical rivers globally and identifies major gaps and unanswered research questions as a part of the igcp 582 on tropical rivers the participants of this project are continuing to tackle many of these issues the collection of papers in this special issue is an outcome of this activity and presents new data on processes and alluvial stratigraphy of the tropical rivers from china india south america and northern australia

mid late pleistocene osl chronology in western amazonia and implications for the transcontinental amazon pathway abstract the origin of the transcontinental amazon drainage system remains unrevealed sedimentary deposits formed from the neogene in the amazonas and solimoes basins constitute natural archives for reconstructing this event in space and time however paleoenvironmental and chronological analyses focusing on these deposits or even their basic mapping are still scarce to allow such investigation in this context primary interests are fluvial strata related to the lithostratigraphic ica formation mapped over a widespread area in western amazonian lowlands although long regarded as plio pleistocene in age this unit has not yet been dated and its overall depositional setting remains largely undescribed the main goal of the present work is to contribute for improving facies analysis and chronology of these deposits approaching an area in southwestern amazonia and another in northern amazonia which are located more than 1000 km apart despite this great distance the sedimentological and chronological characteristics of deposits from these two areas are analogous hence facies analysis revealed paleoenvironments including active channel abandoned channel point bar crevasse splay and floodplain which are altogether compatible with meandering fluvial systems similarly optically stimulated luminescence osl dating revealed thirty three ages ranging from 65 4 16 9 to 219 6 25 1 ky in addition to three outliners of 54 0 7 6 337 3 36 9 and 346 6 48 6 ky and nine 97 1 9 9 to 254 8 23 8 ky for the areas in southwestern and northern amazonia respectively these data lead to establish that deposits mapped as ica formation over a vast area of western brazilian amazonia have a mid late pleistocene age rather than the previously inferred plio pleistocene age it follows that if plio pleistocene deposits exist in this region they remain to be dated and must be restricted to a narrow belt in western amazonia as well as isolated occurrences underlying the mid late pleistocene strata characterized herein the combination of data from this work with previously published provenance studies supports main andean sediment sources only in the mid late pleistocene it is proposed that before this time the amazon river was restricted to eastern amazonia being separated from western amazonian drainage basins due to the presence of the purus arch erosion and or subsidence of this geological feature would have promoted the connection of these drainage systems ultimately with the expressive record of the transcontinental amazon pathway into the atlantic ocean in the mid late pleistocene

palaeodrainage on marajo island northern brazil in relation to holocene relative sea level dynamics a large area in northeastern marajo island northern brazil has been characterized geomorphologically applying information acquired from landsat imagery this study was combined with detailed sedimentologic analysis of continuous cores which provided a record of depositional settings developed in this area through the holocene the results revealed well preserved meandering to anastomosed drainage networks of wide palaeochannels that were superimposed by a narrower palaeochannel system in both cases the sedimentary record consists of sands heterolithic deposits and muds locally rich in plant debris the strata are organized into fining upward successions that reach approximately 18 m thick in the wide channels and 4 m thick in the narrow channels sedimentary features suggestive of a coastal location for the wider palaeochannels and reworking of sediments by tidal currents include the prevalence of well to moderately sorted rounded to sub rounded fine to medium grained sands displaying foreset

late quaternary sedimentary dynamics in western amazonia implications for the origin of open vegetation forest contrasts abstract this work investigated the evolution of sedimentary environments during the latest quaternary and their influence on the paradoxical occurrence of open vegetation patches in sharp contact with the amazonian forest the approach integrated pre existing geological and floristic data from lowlands in the brazilian amazonia with remote sensing imagery including multispectral optical images tm etm and aster phased array l band synthetic aperture radar palsar insar c band srtm dems and high resolution images obtained from google earth the detection of an abundance of paleomorphologies provided evidence of a scenario in which constant environmental shifts were linked to the evolution of fluvial and megafan depositional systems in all studied areas the open vegetation patches are not random but associated with sedimentary deposits representative of environments either deactivated during the holocene or presently in the process of deactivation sedimentary evolution would have determined the distribution of wetlands and terra firme in many areas of the amazonian lowlands and would have a major impact on the development of open vegetated patches within the modern rainforest subsiding areas were filled up with megafan deposits and many fluvial tributaries were rearranged on the landscape the close relationship between vegetation and the physical environment suggests that sedimentary history related to the evolution of depositional settings during the latest quaternary played a major role in the distribution of flooded and non flooded areas of the amazonian lowlands with a direct impact on the distribution of modern floristic patterns as the depositional sites were abandoned and their sedimentary deposits were exposed to the surface they became sites suitable for vegetation growth first of herbaceous species and then of forest although climate fluctuations might have been involved fault reactivation appears to have been the main cause of changes in depositional dynamics through time a process that had an immediate effect on the development of large open vegetation patches intermingled with the amazonian rainforest

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

late holocene tectonic influence on hydrology and vegetation patterns in a northern amazonian megafan abstract the factors that control the heterogeneous canopy of the amazonian rainforest have long been a topic of interest for research among all hypotheses changes in landscape due to geological processes have been increasingly defended large areas of open vegetation in the northern amazonian lowlands are confined to paleolandforms created by the abandoning of megafan depositional systems previous investigations related the megafan sedimentation in this region to late pleistocene holocene tectonic reactivations however the influence of neotectonics on both the amazonian megafans and associated vegetation cover remains to be further investigated we studied a depression preliminarily noticed in the south central sector of one amazonian megafan i e virua megafan aiming to determine if its genesis was due to recent tectonic subsidence after the megafan abandoning the investigation combined morphostructural hydrological and remote sensing reflectance analyses as well as characterization of floristic communities in the modern and ancient environments based respectively on field inventories and palynological data the results indicated a nearly 10 km long and rectangular shaped area having straight margins paralleling nne ssw trending regional structural lineaments hydrological analysis suggests that this area experiences the largest floods during wet seasons a condition compatible with the topographic data that revealed a smoother gradient of 3 m comparing this area with others from the megafan plain in the studied depression numerous forest patches are arranged as sets of parallel straight lines trending consistently in the ne sw direction the patches have spectral values that differ significantly from other forest patches over the megafan surface being compared to those of seazonally flooded forests such as igapos and varzeas that surround the megafan paleolandform in addition a forest patch within the depression revealed trees physiologically adapted to tolerate submersion 6 to 7 months per year which is also a characteristic of the inundated forests external to the megafan these data altogether led us to conclude that the studied depression consists of a shallow subsiding basin formed by tectonic reactivations in the mid late holocene tectonic instability at around 2 cal kyr bp further disturbed this system by creating ne sw trending lakes and leading to the replacement of seazonally flooded forests by grasslands thus tectonic activity in a relatively recent geological time must be accounted when analyzing plant distribution in the amazonian wetlands

combined dating of goethites and kaolinites from ferruginous duricrusts deciphering the late neogene erosion history of central amazonia abstract this study focuses on the dating of millimetric pisoliths from ferruginous duricrusts located in central amazonia brazil by coupling detailed mineralogy and two relevant dating methods i u th he analysis of goethites by mass spectrometry ii analysis of radiation induced defects of kaolinites embedded in duricrusts by electron paramagnetic resonance spectroscopy epr three samples collected at different elevations in the landscape were selected the goethite corrected ages range from 1 1 15 2 ma and consistently increase with elevation epr was performed on kaolinites after several cycles of a deferration procedure considering extreme geochemical scenarii the ages of kaolinites are 1 7 7 1 ma for a closed system and 4 0 16 7 ma for an open system with 100 rn loss ages of goethites and kaolinites are close in the limit of the uncertainty of the methods they show important periods of duricrust formation at middle miocene and at late miocene pliocene data allow the estimation of lowering rates between 3 and 8 m ma in agreement with independent data available in the literature for the central amazonia region or even for erosion of other cratons in the world the two dating methods can be compared on common duricrust samples by revealing single or multi step weathering erosion episodes related to relictual paleosurfaces they also bring promising contributions to an advanced knowledge of ancient and complex lateritic geosystems

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

recycling of amazon floodplain sediment quantified by cosmogenic 26al and 10be the modern amazon trunk stream and tributaries contain substantial amounts of sediments that were formerly stored for 1 3 m y in floodplains by reworking of floodplain deposits this sediment is being incorporated into amazonian streams we conclude this from using the ratio of in situ produced cosmogenic 26 al to 10 be in detrital sediment from rivers draining the sediment producing andean guyana and brazilian shield areas as well as the largest amazonian lowland rivers samples from the modern amazon river yield 26 al 10 be ratios between 3 8 and 5 5 and thus represent mixing between a non buried sediment end member and a formerly buried sediment end member the non buried end member is sourced in the andes with a ratio of 6 5 reflecting recent erosion of hillslopes exposed to cosmic rays and the buried end member is potentially represented by deeply stored floodplain sediment as old as miocene age with a ratio of 3 that was partially shielded from cosmic rays this simple picture of binary mixing is complicated by grain size effects coarser grained sediment from non andean settings lowlands cratonic shields contains slightly more formerly buried material whereas finer grained lowland sediment mostly records non buried 26 al 10 be ratios and nuclide concentrations that are indicative of an andean source our assessment shows an incorporation of formerly buried old sediment of central amazonian floodplains to the fresh andean derived sediment mainly carried in the amazon trunk stream we interpret our results to mean that the amazon regularly avulses into these old deposits that crop out in more distal vicinities to the main river and temporarily leaves its current holocene channel

subduction of continental lithosphere in the banda sea region combining evidence from full waveform tomography and isotope ratios article i nfo we provide new insight into the subduction of old continental lithosphere to depths of more than 100 km beneath the banda arc based on a spatial correlation of full waveform tomographic images of its lithosphere with he pb nd and sr isotope signatures in its arc volcanics the thickness of the subducted lithosphere of around 200 km coincides with the thickness of precambrian lithosphere as inferred from surface wave tomography while the deep subduction of continental material in continent continent collisions is widely recognised the analogue process in the arc continent collision of the banda region is currently unique the integrated data suggest that the late jurassic ocean lithosphere north of the north australian craton was capable of entraining large volumes of continental lithosphere the banda arc example demonstrates that continental lithosphere in arc continent collisions is not generally preserved thus increasing the complexity of tectonic reconstructions in the particular case of timor the tomographic images indicate that this island is not located directly above the northern margin of the north australian craton and that decoupled oceanic lithosphere must be located at a considerable distance north of timor possibly as far north as the northern margin of the volcanically extinct arc sector the tomographic images combined with isotope data suggest that subduction of the continental lithosphere did not lead to the delamination of its complete crust a plausible explanation involves delamination within the continental crust separating upper from lower crustal units this interpretation is consistent with the existence of a massive accretionary complex on timor island with evidence from pb isotope analysis for lower crust involvement in arc volcanism and with the approximate gravitational stability of the subducted lithosphere as inferred from the tomographic images the subduction of continental lithosphere including crustal material beneath the banda arc indicates that global injection rates of continental material into the mantle may have been underestimated and that continental subduction in arc continent collisions may have contributed more to isotopic signatures of earth s mantle than previously recognised

partial anhysteretic remanence and its anisotropy applications and grainsize dependence the distribution of remanence coercivities in a rock sample can be obtained by differentiating alternating field af demagnetization curves of anhysteretic remanent magnetization arm however significant errors can be introduced by grain interactions and by the noise inherent in differentiation we have found that 1 this error can be significantly reduced by measuring partial arm s parm s and 2 parm patterns show a strong empirical correlation with grain size parm s are imparted by exposing a sample to a small direct magnetic field in the presence of a large decreasing af the direct field is switched on over a specified range of af s thus magnetizing a fraction of the magnetite grains in the corresponding coercivity range we have further found in both synthetic and natural sediments that measurement of the anisotropy of parm enables us to detect the preferred orientations of distinct subpopulations of magnetite particles related to the size and shape of the grains

thermal regimes anatexis and orogenesis relations in the western lachlan fold belt southeastern australia abstract crustal evolution of the lachlan fold belt of southeastern australia has involved the mobilisation of significant melt volumes now preserved as granites suggesting that large parts of the lower crust were at t 800 c for part of the thermal evolution this anomalous thermal activity has been variously related to continental extension magmatic underplating and to thinning of the mantle lithosphere overriding considerations reflected in the surface geology must include the tectonic setting the thermal regime and the state of stress in the crust at the time of anatexis one dimensional geothermal modelling suggests that no one mechanism can generate the required thermal regime necessary to produce the volume of granite observed in the crust the observed geological relationships do not match those predicted for lithospheric thinning due to continental extension magmatic underplating can cause melting but must involve extensive source zones and allow continual feeding of the magma to higher crustal levels the crustal thickening lithospheric thinning mechanism involves upwelling of hot asthenospheric mantle to the base of the crust but the melt products are confined to a very thin layer at the crust mantle boundary mechanisms for melt extraction and emplacement to higher crustal levels must be explained in the western lachlan fold belt the presence of a major mid crustal detachment fault ductile shear zones in the lower crust and marked internal shortening of the upper and possibly lower crustal blocks suggests that shear heating may also provide a significant component to the heat budget of the evolving crust melting will not occur however unless thermal equilibration of both the upper and lower plates takes place prior to shear heating such equilibration will involve time scales in the order of 30 50 ma unless extra heat is added into the lower crust e g advective input inferred flexural rigidity of the lithosphere related to basinal subsidence during the period of orogenesis in central victoria suggests that thermal effects related to granitoid intrusion must be largely post tectonic and that felsic intrusive activity is a consequence of the crustal thickening rather than the cause thermal regimes during orogenesis cannot be restricted to a single mechanism upwelling of hot asthenosphere causing significantly higher heat input at the base of the crust coupled with shear heating and internal strain heating of the crust during the crustal thickening stage as well as thermal effects related to magma intrusion into the lower crust result in a complex thermal history during collisional orogenesis

d rossetti p mann de toledo a m goes new geological framework for western amazonia brazil and implications for biogeography and evolution quaternary research 63 2005 78 89 letter to the editor

floodplain channel morphology and networks of the middle amazon river 1 floodplain channels are important components of river floodplain systems and are known to play a key role in hydrodynamic exchange and sediment transport the amazon floodplain exhibits complex networks of these channels and despite their potential importance to this globally important wetland system these floodplain channels are relatively unstudied the research presented here is the first systematic and detailed study of the network and morphologic characteristics of a large number of these channels in the middle reach of the central amazon river using analysis of data derived from landsat enhanced thematic mapper plus etm mosaic and field survey our findings show that the channels are ubiquitous their width varies widely and some of their characteristics can be fitted using power laws potentially much like the self similar or fractal like behavior hypothesized for other types of fluvial networks in all 96 of the floodplain channels are not wide enough to be represented well or at all in the 90 m shuttle radar topography mission data channel depths are tied closely to the local amplitude of the passing main river flood wave p value of 0 75 except where there are local runoff inputs which results in substantially deeper channels which provide preferential flow paths across the floodplain channel networks imply that areas of the floodplain function for large parts of the flood cycle as separate hydrogeomorphic land units here termed floodplain hydrological units fhus these hypothesized fhus also have distinct spatial and pattern characteristics and it is suggested here that their differences could provide the beginnings of a framework for understanding the detailed hydrodynamics of the floodplain in particular different types of fhus have differences in flood water source which will have important implications for biogeochemical studies of the wetlands

geothermal and rheological implications of intracontinental earthquakes beneath the kanto tokai region central japan abstract the kanto tokai region representing a boundary zone of three lithosphere plates has been the subject of intense geological geophysical studies for decades the increased attention can be explained partly by the complicated tectonic situation and even to a greater extent by the permanent threat of earthquakes in the region in this work the rheology of the lithosphere in the area is estimated on the basis of frictional failure in the brittle regime and power law steady state creep in the ductile regime rheological models are constructed for a three layered model involving a granitic upper crust a granulitic or gabbroic lower crust and a dunitic upper mantle results show a marked rheological stratification of the lithosphere controlled primarily by the thermal regime in zones of very low surface heat flow 40 mw m 2 the lithosphere shows brittle features down to depths of 50 60 km at least in areas of normal to moderately high heat flows 50 90 mw m 2 a soft ductile zone at the depth levels of the lower crust sandwiched between two brittle layers appears at locations with very high heat flows 100 120 mw m 2 only the upper crust remains brittle and plastic flow of rocks may start here from the depths of 10 15 km this rheological stratification of the lithosphere is compared with the depth distribution of earthquakes in the area and a generally reasonable confinement of seismicity to the brittle levels is indicated

studies on holocene mangrove ecosystem dynamics of the braganca peninsula in north eastern para brazil abstract three sediment cores from the braganca peninsula located in the coastal region in the north eastern portion of para state have been studied by pollen analysis to reconstruct holocene environmental changes and dynamics of the mangrove ecosystem the cores were taken from an avicennia forest bosque de avicennia bda a salt marsh area campo salgado cs and a rhizophora dominated area furo do chato pollen traps were installed in five different areas of the peninsula to study modern pollen deposition nine accelerator mass spectrometry radiocarbon dates provide time control and show that sediment deposits accumulated relatively undisturbed mangrove vegetation started to develop at different times at the three sites at 5120 14c yr bp at the cs site at 2170 14c yr bp at the bda site and at 1440 14c yr bp at the fdc site since mid holocene times the mangroves covered even the most elevated area on the peninsula which is today a salt marsh suggesting somewhat higher relative sea levels the pollen concentration in relatively undisturbed deposits seems to be an indicator for the frequency of inundation the tidal inundation frequency decreased probably related to lower sea levels during the late holocene around 1770 14c yr bp at bda around 910 14c yr bp at fdc and around 750 14c yr bp at cs the change from a mangrove ecosystem to a salt marsh on the higher elevation around 420 14c yr bp is probably natural and not due to an anthropogenic impact modern pollen rain from different mangrove types show different ratios between rhizophora and avicennia pollen which can be used to reconstruct past composition of the mangrove in spite of bioturbation and especially tidal inundation which change the local pollen deposition within the mangrove zone past mangrove dynamics can be reconstructed the pollen record for bda indicates a mixed rhizophora avicennia mangrove vegetation between 2170 and 1770 14c yr bp later rhizophora trees became more frequent and since ca 200 14c yr bp avicennia dominated in the forest

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

provenance of quaternary and modern alluvial deposits of the amazonian floodplain brazil inferred from major and trace elements and pb nd sr isotopes abstract chemical and pb nd and sr isotope data from floodplain sedimentary deposits in the solimoes river northwest brazil constrain the variability of the deposits through the quaternary and help reconstruct the paleogeography of amazonia since the uplift of the andes compared to the modern alluvial deposits in the active channel the quaternary terrace deposits in the solimoes river have a higher chemical index of alteration cia higher sio2 al2o3 ratios higher concentrations of immobile elements higher radiogenic sr isotopic compositions more negative end t 0 values and older nd tdm ages these geochemical characteristics indicate that the quaternary sediment sources were composed by more felsic and or recycled old continental crust derived rock materials than the younger source areas which contributed major arc volcanic materials to the flood plain deposits this shift in the composition of the sediment source areas indicates that a significant reorganization of the architecture of the solimoes river has occurred since the uplift of the andes

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

dating kaolinite from the neogene ica formation and overlying laterites central amazonia brazil constraints for a stratigraphic correlation abstract the neogene ica formation is a major sedimentary deposit in central amazonia brazil that is critical to the understanding and modeling of the evolution of the amazon basin however its chronostratigraphy is poorly defined mostly because it is not fossiliferous and it does not contain volcanic ash levels this study is the first attempt at the absolute dating of the sediments and overlying soils of the ica formation by electron paramagnetic resonance epr of radiation induced defects in kaolinite rids four profiles of unconsolidated sediments overlaid by lateritic soils from the barcelos region amazonia brazil were studied the concentration of rids was assessed from epr spectra through a simplified procedure on a selected set of samples excluding those showing too strong of interference from the signal of trace vanadyl complexes it is shown that in such sedimentary material both the microscopic distribution of thorium together with uranium and the origin of kaolinite either detrital or authigenic are critical for kaolinite dating considering an argued authigenic origin and two scenarios of th distribution it appears that most of the present chronologic data support the stratigraphic correlation with the madre de dios sedimentary formation in peru previously proposed in the literature for the ica formation 9 5 2 5 ma this study also shows a rejuvenation process of kaolinite both in the uppermost soil and at depth of the profiles this rejuvenation occurred during paleoclimatic periods already revealed in other weathering covers from the amazon basin and possibly resulted from downward weathering in the topsoil and water table or lateral weathering at depth

late quaternary vegetation dynamics in the southern amazon basin inferred from carbon isotopes in soil organic matter abstract carbon isotopes of soil organic matter som were used to evaluate and establish the chronology of the vegetation dynamics of an ecosystem presently composed of savannas surrounded by forests the study was carried out on a 200 km transect along highway br 319 on the border of amazonas and rondonia states in southern amazon brazil large ranges in 13 c values were observed in som collected from profiles in the savanna 27 to 14 and forest regions 26 to 19 reflecting changing distribution of 13 c depleted c 3 forest and 13 c enriched c 4 savanna vegetation in response to climate change these results indicate that from about 17 000 to 9000 14 c yr b p the study area was covered by forest vegetation between approximately 9000 and 3000 14 c yr b p savanna vegetation expanded at the expense of the forest although the expansion of savanna did not occur with the same intensity along the study transect this process was very clearly registered by 13 c enrichment in the som since 3000 14 c yr b p the carbon isotope data suggest that forested regions have expanded this study adds to the mounting evidence that extensive forested areas existed in the amazon during the last glaciation and that savanna vegetation expanded in response to warm and dry conditions during the early to middle holocene

neotectonics and tree mortality in a forest ecosystem of the negro basin geomorphic evidence of contemporary seismicity in the intracratonic brazilian amazonia abstract neotectonic activity was increasingly recorded over the amazonian lowlands even during historical times of a few tens of centuries however detailed analyses linking structures and styles of deformation are still few considering the large dimension of the region such events in a relatively recent time are expected to have caused conspicuous impacts on amazonian ecosystems and forest disturbance changes in topography and hydrology by fracturing faulting and folding during either subsidence or uplifting are direct effects on landscape that leave marks on the structure of the current forest such as tree mortality and community succession two rectangular elbow like shaped wetlands from the left margin of the middle negro river valley in northwestern amazonia were attributed to neotectonics remote sensing imagery was applied to test this hypothesis and also provide a tectonic model that can explain deformation dynamics in this area the studied wetlands and adjacent river systems display various morphostructural anomalies compatible with a tectonic control as well as lineaments paralleling main nw and ne trending regional structures the geomorphic analysis led to suggest that the wetlands are depressions formed by nw and ne trending master boundary faults of horizontal displacements intercepted by various subsidiary faults transtensional strike slip regime is recorded by both left and right lateral faults with the wetlands corresponding to subsiding areas owned to conjugate strike slip faults habitat fragmentation and other ecological processes promoted by tectonic deformation would have impacted the overlying forest canopy by changing its structure due to tree mortality the complex compartmentalization imposed by active tectonics would have exposed the ground to contrasting hydrological conditions which controlled the rate of tree mortality within the wetlands we pose that the tectonic disturbance and associated tree mortality documented in the middle negro river evidence contemporary seismicity within the intracratonic brazilian amazonia

environmental magnetism a practical guide j walden f oldfield j and smith eds technical guide no 6 quaternary research association london 1999 243 pp price 17 00 9 00 to qra members isbn 0 907780 42 3

influence of low amplitude high frequency relative sea level changes in a wave dominated estuary miocene sao luis basin northern brazil abstract miocene deposits exceptionally well exposed along several coastal cliffs between the towns of alc ntara and guimaraes maranhao state northern brazil record the history of low amplitude high frequency sea level changes in a wave dominated incised valley estuarine setting these deposits are interpreted as estuarine in origin based on i depauparated ichnological assemblage and occurrence of gyrolithes both typical of highly stressed brackish water conditions ii dominance of sedimentary structures diagnostic of tidal processes e g alternating thicker thinner sand bundles marked by reactivation surfaces and or mud drapes and iii presence of widespread channel deposits this estuarine fill consists of three stratigraphic units units 1 3 1 unit 1 shoaling shoreline sh and tidal channel ch 2 unit 2 shoaling shoreline sh tidal channel ch facies associations lagoon lg and flood tidal delta washover ftd w facies associations and 3 unit 3 tidal channel ch estuarine bay eb and fluvial influenced channel fch facies associations the individual units internally show a dominantly prograding character a pattern also reflected throughout the succession by the overall upward stacking of units showing successively more restricted conditions these characteristics combined with the position in the uppermost portion of the valley fill succession led to suggest that the deposits exposed in the study area likely formed during the transition from rising to highstand stages of relative sea level mapping of regionally significant discontinuity surfaces led to the recognition of a sequence boundary i e discontinuity surface ds1 marked by erosion and subaerial exposure at the valley floor this surface separates the miocene succession from underlying cretaceous rocks and it is attributed to a major period of incision that led to the genesis of the incised valley during relative sea level fall unit 1 onlaps against ds1 and is bounded above by discontinuity surface ds2 which probably formed by tidal ravinement during a period of transgressive erosion this ravinement surface is the boundary between two shoaling up successions i e units 1 and 2 interpreted as parasequences from a same depositional sequence discontinuity surface ds3 between units 2 and 3 is attributed to a sequence boundary marked by significant erosion and subaerial exposure and is interpreted as a reflex of a renewed period of incision within the estuary due to a strong drop in relative sea level the top of unit 3 is defined by another discontinuity surface i e ds4 with pronounced erosion and subaerial exposure which is attributed to a sequence boundary that ended the estuarine deposition in the study area thus unit 3 defined at the base and top by unconformities is a complete depositional sequence resulting from a low amplitude high frequency relative sea level cycle superposed upon the overall highstand stages of the estuary evolution

reconstruction of the miocene western amazonian aquatic system using molluscan isotopic signatures abstract strontium isotopic ratios and sr contents of molluscs from the miocene pebas formation in colombian amazonia indicate that the bulk of the pebas formation was deposited in a freshwater environment predominantly draining the andes with minor contribution from the shield regions the molluscan oxygen and carbon isotopic composition does not provide a palaeosalinity proxy but points to a lacustrine palaeoenvironment the faunal character confirms such an interpretation sr isotopic ratios further suggest that a marine incursion that reached the area was composed of a mixture of shield derived freshwater and seawater with a maximum salinity of about 5 psu faunistic components of this incursion level support a shield seawater mixing the incursion most likely reached western amazonia from the north probably along the western guyana shield

holocene environmental changes from the rio curua record in the caxiuana region eastern amazon basin abstract holocene environments have been reconstructed by multiproxy studies of an 850 cm long core from rio curua dating to 8000 14 c yr b p the low energy river lies in the eastern amazon rain forest in the caxiuana national forest reserve 350 km west of belem in northern brazil sedimentological mineralogical and geochemical dates demonstrate that the deposits correspond to two different environments sediments of an active river before 8000 14 c yr b p and later a passive river system the pollen analytical results indicate four different local and regional holocene paleoenvironmental periods 1 a transition to a passive fluvial system and a well drained terra firme unflooded upland amazon rain forest with very limited development of inundated forests varzea and igapo 7990 7030 14 c yr b p 2 a sluggish river with a local mauritia palm swamp and similar regional vegetation as before 7030 5970 14 c yr b p 3 a passive river forming shallow lake conditions and with still abundant terra firme forest in the study region 5970 2470 14 c yr b p and 4 a blocked river with high water levels and marked increase of inundated forests during the last 2470 14 c yr b p increased charcoal during this last period suggests the first strong presence of humans in this region the atlantic sea level rise was probably the major factor in paleoenvironmental changes but high water stands might also be due to greater annual rainfall during the late holocene

miocene paleoenvironmental changes in the solimoes basin western amazon brazil a reconstruction based on palynofacies analysis abstract we report palynofacies from two sections of miocene solimoes formation of the solimoes basin western amazon brazil to better understand paleoenvironmental evolution based on cluster analysis five palynofacies associations were recognized dominated by either opaque non opaque algae miospores and structureless marine organic matter the distribution of palynofacies associations reflects a complex depositional system with fluvial lacustrine estuarine and shallow marine environments a high abundance of woody material indicates a fluvial lacustrine dominated environment three marine incursions were recognized indicated by the presence of dinoflagellate cysts and microforaminifera test linings marine influence was more evident during the late middle late miocene interval when transgressions occurred over a greater extent of the amazon region

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

astronomical forcing of contrasting rainfall changes in tropical south america between 12 400 and 8800 cal yr b p abstract today precipitation over tropical south america is largely controlled by the seasonal movements of the inter tropical convergence zone itcz during the summer the itcz is shifted southward due to the warming of the continent paleoclimate data from southeastern amazonia and the central andes indicate that these two areas evolved similarly during the last 30 000 yr however between 12 400 and 8800 cal yr b p eastern amazonia received substantial moisture whereas the bolivian altiplano was arid this suggests that the itcz during summer was then farther north than it is today

applying dem srtm for reconstructing a late quaternary paleodrainage in amazonia abstract remote sensing is a particularly invaluable tool that has helped the detection of paleomorphologies produced by river dislocation in a variety of landscapes which has contributed in reconstructing the geological evolution of many fluvial systems this technique might provide useful information to discuss the evolution of large fluvial systems in special those located in areas of difficult access where the acquisition of field data is difficult application of remote sensing for paleodrainage characterization in densely vegetated tropical areas is scarce in the literature this work records processing of the digital elevation model dem derived from the shuttle radar topography mission srtm which succeeded in revealing an ancient drainage complex of the madeira river one of the main amazonas tributaries where other remote sensing products failed the detection analysis of this paleodrainage and of its modern counterpart within the geological framework available for this region leads to propose that activity along pre existent faults during the latest quaternary would have promoted the southeastward dislocation of a nearly 200 km long segment of the madeira river during this process an impressive paleodrainage network was left behind which was only able to be detected using the dem srtm application of this technique might be of great help to the detection of paleodrainage morphologies in densely vegetated areas similar to the amazonas lowland the dynamics of channel migration in this and many other large scale tropical river systems might benefit from the investigation based on data derived from dem srtm

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

magnetic susceptibility mapping of the sudbury area ontario canada evaluating pollution distributions decades later this study evaluates the use of magnetic susceptibility and magnetic parameter measurements in assessing spatial and temporal variations of pollutants that emanated from mining industries in and around sudbury ontario canada for this purpose in situ magnetic susceptibility in situ was measured at 106 sites on a grid of 10 km 10 km and 5 km 5 km the in situ values ranged from 2 10 5 to 149 10 5 si and the highest in situ values were observed near the active copper cliff and inactive coniston mining sites the lowest in situ values were measured at increased distances from possible pollution sources therefore mapping of in situ magnetic susceptibility values is a proxy to polluted areas in and around sudbury to evaluate potential anthropogenic and or lithogenic input to in situ low frequency mass specific magnetic susceptibility lf variations with depth were classified into four different types of profiles for further investigation of magnetic minerals in the samples labo

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

the role of tectonics in the late quaternary evolution of brazil s amazonian landscape tectonic reactivation has long been suggested to have occurred over several areas of the amazonian lowland in the neogene holocene numerous landscape changes documented in this region particularly variations in fluvial dynamics and morphologies have nevertheless been exclusively attributed to climatic fluctuations minimizing the effect of tectonics over the neogene holocene evolution of the amazon basin can result in a distorted interpretation of the sedimentary record and as a consequence equivocal paleoclimatic reconstructions climate has often prevailed as a hypothesis to explain changes in fluvial dynamics possibly due to the still scarce synthesizing publications focusing exclusively on the neotectonic influence over this region the present work provides a review of existing data focusing on quaternary tectonics over a large area of the amazonian lowland the existing information is complemented by original morphostructural data on quaternary terrains from this region which were acquired through remote sensing the goals were to analyze the geographic extent of late quaternary deposits and to determine the impact of tectonics on their development and on the latest evolution of the amazon drainage basin the analysis shows that 1 the amazonian lowland experienced geographically widespread sedimentation in the late pleistocene holocene 2 sedimentation was promoted by the creation of new depositional sites mostly likely resulting from tectonic activity and 3 tectonic reactivation impacted the latest evolution of this entire region with particular influence on drainage basins as revealed by river courses with an abundance of morphostructural lineaments evidencing fault control hence the nw ne nearly n s and to a lesser extent w e morphostructural trends recorded in all areas investigated here are generally associated with strike slip deformation in addition they are conformable with the main orientation of tectonic structures from adjacent crystalline rocks of the precambrian basement they also match tectonic trends documented in neogene and younger sedimentary deposits from this and several other areas of northeastern brazil furthermore some are detected locally in the subsurface by geophysical data furthermore a profusion of morphostructural anomalies is imprinted on the surface of neogene holocene deposits of the amazonian lowland the main ones consisting of 1 highly asymmetric modern drainage basin still undergoing organization with anomalous patterns i e trellis sub dendritic sub parallel and rectangular that often vary from one another frequent river captures and rivers flowing in contrary directions along the time and straight channels that form triangular shapes or triple junctions or with local meandering and subtle enlargements 2 an abundance of channels floodplains lakes and entire valleys with straight and orthogonal margins that locally form rhombic shapes or which may be laterally displaced 3 fluvial rias i e rivers that became enlarged as their mouth were barred bounded by sharp and orthogonal lineaments which result in rectangular shapes 4 termination of megafans into rhombic shaped depressions bounded by straight lineaments or their lateral displacement several kilometers apart and 5 rivers with asymmetric terraces laterally displaced by faults these observations together are consistent with a landscape controlled by neotectonic activity the modern seismogenic record which reveals even high magnitude earthquakes indicates active deformation over the amazonian lowland at the present time the neotectonic history of the amazonian lowland is explained within the context of an overarching mechanism most likely related to intraplate stresses this was probably caused by reactivation of pre existing structures since the neogene that was most likely promoted by activity along oceanic fracture zones and transform faults of the brazilian equatorial margin as well as tectonism in the andean region

geothermal models and mantle rheology in australia abstract geothermal models can be constructed for australia consistent with seismic refraction data variations in velocity can be assumed to coincide with variations in thermal conductivity and heat production for discrete blocks representing a complex geological section uniform physical properties are readily assigned at each depth conforming on average to conventional estimates based on an exponential decay errors are assessed assuming random perturbations in each segment subject to constant boundary conditions a range of 20 in heat flow for any province can generate variations of 160 c at depths of 50 km consequently several different rheological models may be required for each craton precambrian cratons can be distinguished from the more recent terranes in eastern australia in terms of the homologous temperature actual temperature melting temperature areas of high heat flow are characterised by plastic flow at depths of 45 15 km consistent with concepts of crustal evolution involving underplating and intrusion in eastern australia seismic events are concentrated in the brittle outer crust of the phanerozoic but there is no stress sufficient to fracture the cool precambrian crust mantle sources incorporating plastic deformation for enhanced thermal conductivity can also be used to explain inflections in the geothermal gradient calculated from xenolith data

holocenic proxies of sedimentary organic matter and the evolution of lake arari amazon region abstract four sediment cores were sampled from lake arari located on marajo island at the mouth of the amazon river the island s vegetation cover is composed mainly of amazon coastal forest herbaceous and varzea vegetation the integration of data on sedimentary structures pollen carbon and nitrogen isotope records c n ratios and radiocarbon ages allowed the identification of changes in vegetation and the sources of organic matter accumulated in the lake during the holocene the data indicate a relatively high flow energy marine water influence and the presence of mangroves during the lagoon phase between 8990 and 8690 cal yr b p and 2310 2230 cal yr b p between 2310 and 2230 cal yr b p and 1000 cal yr b p the flow energy decreased and the mangroves were replaced by herbaceous vegetation following the decline in marine influence likely due to the increase in freshwater river discharge during the last 1000 years lake arari was established in association with the expansion of herbaceous vegetation and the dominance of freshwater algae

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

upper mantle shear velocity beneath eastern australia from inversion of waveforms from skippy portable arrays in 1993 the research school of earth sciences of the australian national university commenced a nationwide seismometry project skippy for a comprehensive study of the upper mantle beneath the australian continent we applied a waveform inversion technique to broad band data recorded while the skippy portable arrays were positioned in eastern australia in order to construct a 3 d model of shear velocity in the upper mantle and transition zone beneath eastern australia and the adjacent oceanic regions the skippy data were augmented by data from the permanent seismological observatories in the region the first step of the waveform inversion used involved the matching of the waveforms of fundamental and higher mode rayleigh waves with waveforms synthesized from radially stratified models in the second stage the linear constraints on radial variations in shear velocity were combined in a tomographic inversion for aspherical variations in shear velocity the preferred model reduces the data variance by 90 per cent owing to the dense data coverage structural features with dimensions larger than 250 km laterally and 50 km vertically are resolved the major structural features inferred from the tomographic images are 1 a substantial increase in the thickness of the high velocity lid from the oceanic region to the phanerozoic continental and proterozoic continental regions 2 a pronounced low velocity zone centred at 140 km depth beneath the part of eastern australia often referred to as the tasman fold belt which confirms previous inferences from rayleigh wave dispersion curves 3 a prominent zone of low wave speeds beneath the eastern margin of the continent that coincides with locations of cenozoic volcanism and regions of enhanced heat flow 4 deep low wave speed anomalies beneath both the coral and tasman seas and 5 localized high wave speed perturbations in the mantle transition zone 410 660 km depth beneath eastern australia in particular beneath the new england fold belt and the mount isa block from the images we infer that the eastern boundary of the precambrian shields does not coincide with a sharp seismic contrast unless this boundary is located further to the east than is commonly assumed the structure of the upper mantle and transition zone beneath eastern australia as inferred from the images is more complex than that beneath the precambrian cratons of central and western australia which could be due to subduction beneath the eastern continental margin prior to opening of the tasman sea if correct this interpretation would imply that part of the upper mantle has moved along laterally with the lithosphere during the relatively fast northward motion of the plane

the occurrence of greigite in sediments from loch lomond greigite has been identified in the sediments of loch lomond using x ray diffraction greigite is the dominant magnetic mineral in the oldest sediments recovered while magnetite predominates in the younger sediments which had previously been used for palaeomagnetic secular variation studies a layer of sediment in between these two magnetic mineral regimes has very low magnetic concentrations probably as a consequence of magnetite dissolution associated with sulphide rich pore waters produced at the time of a marine incursion the greigite largely oxidises once the sediment is exposed to air but if freeze dried the greigite becomes surprisingly stable following freeze drying the greigite can be heated to 280 c in air before it alters and loses its strong ferrimagnetic properties

fluvial perturbance in the western amazon basin regulation by long term sub andean tectonics haffer s refuge theory proposes that during the arid climatic phases of the late pleistocene tropical lowland forests of amazonia were reduced to isolated patches contributing to the high species richness of the present day forest the theory was developed because no obvious historic or modern geomorphic isolation barriers were recorded in amazonia analyses of radar images combined with stratigraphical data show that in the basinal forelands of the tectonically active andes the geological setting causes long term fluvial perturbance this leads to a temporally structured highly complex mosaic of fossil and present floodplains these dynamics have been present with varying activity and geographic range during the tertiary and quaternary providing site turnover that has not been recognized by the biogeographic tradition of the amazon basin

a tectonically triggered late holocene seismite in the southern amazonian lowlands brazil abstract definitive criteria for identifying the triggers of soft sediment deformation structures ssdss from sedimentary deposits are still under construction in this work we aimed to contribute to advancing this issue describing the sedimentological characteristics of an interval of ssdss formed in a region of the amazonian lowlands having numerous morphostructural evidences of neotectonic reactivation the deformed interval is 1 5 m thick and consists of load and flame structures interbedded with massive beds sandwiched between well stratified deposits of a fluvial sandy channel succession with a radiocarbon age of 1301 1382 cal yr bp the ssdss are intercepted by numerous small scale normal graded faults the most interesting is that these faults display segmented nw se trending planes that propagate upward after horizontal displacements of a few millimeters processes intrinsic to channel dynamics could not justify this fault morphology which is related to recurrent short lived seismic perturbations with intervening sedimentation following a main tremor a relation to fault reactivation is proposed with basis on the fault trend compatible with the regional structural pattern and the numerous morphostructural anomalies verified in the study area including trellis and sub trellis drainage rectangular and to a less extent annular channels anomalously enlarged channels orthogonal channels and valleys channel captures sudden changes in channel morphology moderate to high basin asymmetry factor af between 10 and 27 moderate to high transverse topography symmetry factor t 0 3 0 5 and anomalous longitudinal river profiles with concavity values 11 to 30 indicative of rivers in process of adjustments the morphology of the fault graded beds described herein might constitute a feature of relevance for identifying seismites in other fluvial deposits

underplating of the crust and xenolith geotherms in australia xenolith data indicate extreme geothermal gradients at shallow depth in eastern australia contrary to results obtained from models based on thermal conduction consequently mechanisms involving partial melting accompanied by underplating must be assumed to supply additional heat unrelated to radiogenic profiles a mid crustal heat source is required to satisfy the xenolith data suggesting a layered structure consistent with episodic underplating at maximum rates approaching 900m myr in eastern australia

late quaternary eolian dust in surficial deposits of a colorado plateau grassland controls on distribution and ecologic effects abstract in a semi arid upland setting on the colorado plateau that is underlain by nutrient poor paleozoic eolian sandstone alternating episodes of dune activity and soil formation during the late pleistocene and holocene have produced dominantly sandy deposits that support grass and shrub communities these deposits also contain eolian dust especially in paleosols eolian dust in these deposits is indicated by several mineralogic and chemical disparities with local bedrock but it is most readily shown by the abundance of titaniferous magnetite in the sandy deposits that is absent in local bedrock magnetite and some potential plant nutrients especially p k na mn and zn covary positively with depth 3 4 m in dune crest and dune swale settings magnetite abundance also correlates strongly and positively with abundances of other elements e g ti li as th la and sc that are geochemically stable in these environments soil property variations with depth can be ascribed to three primary factors 1 shifts in local geomorphic setting 2 accumulation of relatively high amounts of atmospheric mineral dust inputs during periods of land surface stability and 3 variations in dust flux and composition that are likely related to changes in dust source regions shifts in geomorphic setting are revealed by large variations in soil texture and are also expressed by changes in soil chemical and magnetic properties variable dust inputs are indicated by both changes in dust flux and changes in relations among magnetic chemical and textural properties the largest of these changes is found in sediment that spans late pleistocene to early holocene time increased dust inputs to the central colorado plateau during this period may have been related to desiccation and shrinkage of large lakes from about 12 to 8 ka in western north america that exposed vast surfaces capable of emitting dust soil properties that result from variable dust accumulation and redistribution in these surficial deposits during the late quaternary are important to modern ecosystem dynamics because some plants today utilize nutrients deposited as long ago as about 12 15 ky and because variations in fine grained silt sediment including eolian dust influence soil moisture capacity

influence of landscape evolution on the distribution of floristic patterns in northern amazonia revealed by δ13c data the amazonian lowlands include large patches of open vegetation which contrast sharply with the rainforest and the origin of these patches has been debated this study focuses on a large area of open vegetation in northern brazil where d 13 c and in some instances c n analyses of the organic matter preserved in late quaternary sediments were used to achieve floristic reconstructions over time the main goal was to determine when the modern open vegetation started to develop in this area the variability in d 13 c data derived from nine cores ranges from 32 2 to 19 6 but with nearly 60 of data above 26 5 the most enriched values were detected only in ecotone and open vegetated areas the development of open vegetation communities was asynchronous varying between estimated ages of 6400 and 3000 cal a bp this suggests that the origin of the studied patches of open vegetation might be linked to sedimentary dynamics of a late quaternary megafan system as sedimentation ended this vegetation type became established over the megafan surface in addition the data presented here show that the presence of c4 plants must be used carefully as a proxy to interpret dry paleoclimatic episodes in amazonian areas copyright 2012 john wiley sons ltd

a late pleistocene holocene wetland megafan in the brazilian amazonia abstract despite the growing interest in megafans definitions provided for this type of environmental setting have not yet been widely agreed upon a record of sedimentary facies distribution in both space and time including a larger number of analogs is particularly needed for improving megafan facies models this work focuses on a large fan like feature from an amazonian wetland in northern brazil morphological data based on remote sensing as well as sedimentary facies and radiocarbon analyses were integrated to propose that this feature is related to a megafan system active during the late pleistocene to holocene the megafan displays a divergent drainage network gently dipping slope and concave up and convex up longitudinal and transverse profiles respectively near surface deposits correspond to fining and coarsening upward sands related to active channels and overbank sand sheets terminal fan lobes sediments are interbedded with abandoned channel floodplain and lake pond muds morphostructural analyses and drainage anomalies revealed a geological setting affected by reactivation of pre existing faults contemporaneous with sediment accumulation establishment of a megafan system in this wetland most likely occurred within a slightly tectonically subsiding basin under favorable climatic conditions during wet seasons high water discharge would have favored sediment transport from highlands into this depositional site high summer temperatures and drought under a monsoonal regime kept the water levels low the described megafan could serve as an analog for contemporary tropical wetland megafans formed under a monsoonal climate regime

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

the role of late pleistocene holocene tectono sedimentary history on the origin of patches of savanna vegetation in the middle madeira river southwest of the amazonian lowlands abstract savanna patches are features of the amazonian landscape that have been long under intense debate but there are still questions about the main factors that have determined their establishment and evolution within the rainforest matrix in particular their geological substrates were poorly documented the aim of this work is to reconstruct the tectono sedimentary history of four savanna patches in the middle madeira river southwest amazonia and discuss its potential control over the development of the savanna communities the approach consisted in the integration of geomorphological sedimentological chronological and floristic data the results revealed that the savanna substrates are sandy deposits aged between 118 9 and 35 6 ky overlain by late pleistocene to holocene muddier successions

the pan amazonian ucayali peneplain late neogene sedimentation in amazonia and the birth of the modern amazon river system abstract we review the neogene geologic history of lowland amazonia in an attempt to focus attention on areas of agreement as well as areas in dispute in this research arena we reinterpret pre existing hypotheses present new data and discuss new insights intended to support a unified synthesis of the amazon basin as a single sedimentary basin albeit on a vast scale during the late miocene to middle late pliocene we document the ucayali peneplain as an isochronous pan amazonian geologic feature that formed following the early to mid miocene quechua i orogenic phase of andean tectonism peneplanation began possibly as early as 15 ma and terminated abruptly near the beginning of the late miocene quechua ii orogenic event at 9 5 9 0 ma subsequently a thin cover of sediments comprising the madre de dios formation began blanketing most of lowland amazonia excepting only the eastern subandean fold and thrust belt and isolated highlands within the basin the buried peneplain is readily observed in river cutbanks throughout amazonia as the marked often angular ucayali unconformity that separates eroded older often folded faulted and weathered moderately to well consolidated tertiary formations from unconsolidated near horizontal upper neogene deposits the dates of formation of major unconformities and subsequent depositional events at widely separated areas within the andes of bolivia ecuador and peru are coincident with that of the ucayali unconformity and deposition of the madre de dios formation and suggest that the events are linked to a common cause which is interpreted to be the still on going collision between the south american and nazca tectonic plates the madre de dios formation has three members the oldest of which documents a short lived high energy depositional environment followed by a moderate energy depositional environment both occurring at a time when drainage from the basin was unobstructed the upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed disorganized and took place over long distances with extremely low gradients the sedimentology of the madre de dios formation particularly the thick massive beds of clay and the widespread presence of paleodeltas and associated geomorphic features on the amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake lago amazonas or more probably within a complex series of interconnected mega lakes that occasionally united to cover most or all of lowland amazonia to a shallow depth from the latest miocene until 2 5 ma the presence of the ucayali unconformity and the relatively uniform lithostratigraphy basin wide of the fluvial fluviolacustrine and lacustrine sediments of the upper neogene madre de dios formation are consistent with the hypothesis that the amazon basin acted as a single undivided sedimentary basin in the late neogene the biostratigraphic correlation across important modern drainage divides of both micro and macro sized late miocene fossil vertebrates recovered from basal conglomerates of the madre de dios formation and the absence therefrom of fossil vertebrates of any other age is also consistent with this hypothesis two 40ar 39ar dates on ash deposits within the madre de dios formation corroborate the upper miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper pliocene age for the youngest sediments of the formation the modern amazon river drainage system was established in the late pliocene at 2 5 ma by the breaching of the eastern rim of the sedimentary basin as a result of the basin being overfilled or by headward erosion of the lower amazon river or both data on cenozoic mass accumulation rates and the chemistry of terrigenous sediments in the atlantic ocean obtained by ocean drilling project leg 154 ceara rise support the postulated timing of the establishment of the modern amazon river drainage system at 2 5 ma rather than the long held view that this event occurred in the late miocene we discuss the important role of ocean currents and sea level fluctuations on terrigenous mass accumulation rates on the ceara rise the postulated time of formation of the modern amazon river is nearly coincident with the onset of the plio pleistocene glacial climatic regime and the lowest sea level stands since the latest middle miocene this analysis indicates that modern amazonia is a product of terrain development within an erosional regime since 2 5 ma

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

provenance of pliocene and recent sedimentary deposits in western amazonia brazil consequences for the paleodrainage of the solimoes amazonas river abstract integrated data on paleocurrents the morphology of detrital minerals and zircon grains chemical compositions and u pb geochronology reveal that the flow of the modern solimoes amazonas river has changed from west to east since the plio pleistocene this finding is supported by several lines of evidence including paleocurrent directions and detrital mineral assemblages in the ica formation and in recent sediments the ica formation which was most likely deposited during the pliocene has ne and se paleocurrents a high proportion of stable detrital mineral assemblages and u pb zircon ages that we interpreted as being derived from the amazonian craton e g the rondonian san ignacio and sunsas grenvillian geochronologic provinces and neighboring provinces including the neoproterozoic to cambrian brazilian pampean mobile belts a small proportion is derived from the cambrian to silurian famatinian continental arch another source is the precambrian and paleozoic basement from the andes cordillera which includes several metamorphic inliers in colombia peru and bolivia the overlying recent deposits have different provenances and are characterized by a more variable detrital assemblage with zircon grains that are enriched in trace elements and depleted in si and have mesoproterozoic ages in our interpretation the erosion of the iquitos arch after deposition of the ica formation allowed the westward expansion of the solimoes amazonas system in the plio pleistocene

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

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

large barchanoid dunes in the amazon river and the rock record implications for interpreting large river systems abstract the interpretation of large river deposits from the rock record is hampered by the scarcity of direct observations of active large river systems that is particularly true for deep channel environments where tens of meters deep flows dominate these conditions are extremely different from what is found in smaller systems from which current facies models were derived mbes and shallow seismic surveys in a selected area of the upper amazonas river in northern brazil revealed the presence of large compound barchanoid dunes along the channel thalweg the dunes are characterized by v shaped concave downstream crest lines and convex up longitudinal profiles hundreds of meters wide up to 300 m in wavelength and several meters high based on the morphology of compound dunes expected preserved sedimentary structures are broad large scale low angle concave up and downstream cross strata passing laterally and downstream to inclined cosets examples of such structures from large river deposits in the rock record are described in the silurian serra grande group and the cretaceous sao sebastiao and marizal formations in northeastern brazil as well as in triassic hawkesburry sandstone in southeastern australia and the plio pleistocene ica formation in the western amazon all these sedimentary structures are found near channel base surfaces and are somewhat coarser than the overlying fluvial deposits favoring the interpretation of thalweg depositional settings the recognition of large barchanoid dunes as bedforms restricted to river thalwegs and probably to large river systems brings the possibility of establishing new criteria for the interpretation of fluvial system scale in the rock record sedimentary structures compatible with the morphological characteristics of these bedforms seem to be relatively common in large river deposits given their initial recognition in five different fluvial successions in brazil and australia potentially enabling substantial improvements in facies models for large rivers

imaging underwater neotectonic structures in the amazonian lowland previous documentation has proposed tectonic activity from the neogene to holocene as one important control on drainage development in the amazonian lowland however recording tectonic structures in this region has been challenging due to environmental difficulties including the low topography and dense vegetation cover that do not favor natural exposures for this reason the role of tectonics in determining river dynamics morphology and sediment deposition in amazonia has been suggested with caution despite limitations morphostructural lineaments have provided useful information to support fault reactivation in several areas of the amazonian lowland however further documentation is required to map the faults and describe their geometry this approach is essential to better understand the geotectonic model of this region in this work analyses of high resolution optical imagery of three amazonian shallow water lakes provided exceptional views of an abundance of linear features on the beds of the

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

fluvial terraces as quantitative markers of late quaternary detachment folding and creeping thrust faulting in the peruvian huallaga basin abstract in the western huallaga basin located in the peruvian subandes a series of nine fluvial terraces with elevations of up to 160 m above current floodplain level fp were formed against the biabo anticline the terraces are progressively more folded with age and terrace elevation and present growth strata suggesting ongoing folding of the anticline optically and infrared stimulated luminescence osl and irsl dating of fluvial terrace sediments yielded ages of 10 8 0 7 ka for the t2 24 2 2 ka for the t4 and 46 9 ka for the t7 terrace uniform uplift rates of 2 mm a 1 for the past 46 ka were calculated terrace profiles and two new seismic profiles that cross the biabo anticline longitudinally and transversally were used to calculate shortening rates of 2 6 mm a 1 for the anticline over the past 46 ka our calculations took into account potential contributions of detachment thrusting parallel to the biabo fold and reverse tear faulting perpendicular to the fold tip as visible on the seismic sections a maximum uplift rate of 0 62 mm a 1 due to reverse thrusting of the bellavista tear fault for the past 46 ka could have contributed to the overall uplift of the bellavista terraces based on trigonometric relationships a reverse fault slip rate of 0 86 mm a 1 and an age of 137 ka were calculated for this fault in the eastern part of the huallaga basin a series of two fluvial terraces were formed with elevations of 13 15 m and 30 m fp and with osl ages of 6 9 0 7 ka for t1 and 30 2 3 1 ka for t2 the interpretation of a third new seismic profile crossing the study area in combination with field observations and trigonometric relationships shows that the terraces were formed by a complex system of oblique ramp and strike slip faulting resulting in uplift and shortening rates of 1 mm a 1 and 1 96 mm a 1 between 30 2 ka and 6 9 ka respectively and uplift and shortening rates of 2 mm a 1 and 3 70 mm a 1 from 6 9 ka until the present reverse slip of the shapaja oblique ramp that was responsible for uplift of the terraces was calculated at 2 19 mm a 1 for the period 30 2 to 6 9 ka and 4 15 mm a 1 from 6 9 ka until the present measured total slip in combination with the calculated slip rates allowed an age calculation of 181 ka for the shapaja thrust ramp our inferences showed that the studied tectonic structures absorb most of the present day deformation in the huallaga basin the calculated uplift and shortening rates are compatible with published present day crustal movement velocities and miocene to present shortening rates for various studied sites of the tropical subandes and suggest that it is deforming at a relatively uniform rate the timing of sedimentation of the terrace sediments may have corresponded to periods of increased precipitation which occurred coeval with the heinrich events of the northern hemisphere terrace records similar as ours have been described for other tectonically highly active areas in the tropical subandes but not in the tectonically less active amazonian lowlands it is suggested that uplift rates of at least 1 mm a 1 are needed to preserve suborbital climate cycles as individual terraces the tropical subandes could therefore be a new key area to investigate the interactions between tectonics suborbital climate forcing and fluvial response during the late quaternary

heat production in the continental crust part i data converted from seismic velocities and their attempted interpretation abstract when using the simple exponential distribution of radiogenic heat production a with depth z a z a o exp z d the statistical analysis of a large number of heat production profiles converted from the observed seismic velocity vs depth profiles and the experimental knowledge of the relationship between seismic velocity and heat production disclosed certain relations between the logarithmic decrement d value and heat flow tectonic age and crustal thickness the d valuein this respect characterizes the depth rate of the differentiation of the heat production in all tectonic units the d parameter decreases with depth as the next step the more complicated distribution law a z a o exp z d a was applied where a characterizes the quantity of removed heat sources by the processes accompanying the crustal evolution such as remelting metamorphosis leaching by underground waters etc all parameters a o a and d are constant within each particular tectonic province and parameter d does not depend on depth contrary to the former law the amount of removed heat sources a reflects the initial concentration of heat sources and is higher in young tectonic terranes in areas of high heat flow and in regions of thin crust

plus ca change a model for stasis and evolution in different environments abstract the plus ca change model proposes that morphological stasis is the usual response to widely fluctuating physical environments on geological timescales until thresholds are reached the lineages that survive in a more changing environment on geological timescales are those that are relatively inert to each environmental twist and turn in contrast to more sensitive lineages in less changing environments generalists in this long term sense are species with properties that enable them to survive throughout wide environmental fluctuations over geological timescales they are thus related to but distinct from ecological eurytopes the model predicts a tendency for continuous gradualistic evolution on land in the tropics and in the deep sea and for more stasis and occasional punctuations in shallow waters and temperate zones as the vast majority of the fossil record comes from dynamic shallow marine environments it is not surprising that many fossil lineages show approximate stasis and occasional punctuations punctuated equilibrium could be being mistakenly perceived as the overwhelming pattern in the history of life because the environments in which gradualism predominates are rarely preserved in the fossil record the evolutionary pattern in a lineage at a single locality is predicted to be stasis when there is greater environmental fluctuation and net directional change and or more widely fluctuating morphologies when environments are more narrowly fluctuating given the quaternary climate upheavals relatively little evolution may be occurring worldwide at present except for evolution induced by humans such a model is bound to have many exceptions it is only about tendency and relative frequency though few case histories are yet known many existing data sets probably contain clues that can be used in testing the model other relevant aspects of perception are also discussed including descriptive biases and evolutionary reversals

miocene fish faunas from the northwestern amazonia basin colombia peru brazil with evidence of marine incursions new evidence indicates marine influences during the miocene in the northwestern amazonia basin this is the first major survey of the ichthyofauna from this area in the miocene fossil fish remains from taxa such as the dasyatoidea myliobatoidea characiformes siluriformes and sciaenidae are found conspicuous finds were two pharyngeal teeth of stephanodus minimus a species that so far had only ever been found in late cretaceous sediments the fish remains of freshwater brackish and marine taxa are deposited together throughout the area this is thought to be related to marine influences during the miocene these finds agree with earlier studies that suggest episodic marine influences which most likely lasted up to the late miocene and disagree with studies that suggest a connection between the atlantic and the caribbean sea over the south american continent the ichthyofaunas also indicate a warm climate as well as shallow and possibly torrential waters c 1998 elsevier science b v all rights reserved