the mera site revisited ice age amazon in the light of new evidence abstract the mera site is the only fossil pollen and forest bed radiocarbon dated 33 26 ka bp in tropical lowland amazonia here i conclude from the study of the geology geomorphology and paleopedology of the mera site area as well as from new 14c and u th age determination of the organic layers that the pollen sections are not of last glacial age but of middle pleistocene age at least no evidence of last glacial age has been found for the mera fossil forest beds the data from the mera sites therefore should neither be used for climate reconstructions of the last glacial maximum nor for disputing the refugial theories

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

an experimental investigation of mechanisms involved in bed load sheet production and migration 1 field measurements indicate that in gravel bed rivers bed load may not be a one to one response to shear stress but may instead fluctuate a great deal over time for a given flow condition both in flume and field experiments these fluctuations were often associated with migration of low relief bed forms called bed load sheets whereas several studies have described bed load sheets as a consequence of grain sorting little is known about the mechanisms responsible for their production and migration these were investigated in flume experiments a set of 20 experiments was conducted under constant feeding rate conditions with mixtures of different uniform sediments and for slopes varying from 0 8 to 9 except for runs performed in high flow conditions we observed periodic bed load sheet production and migration associated with fluctuations of bed slope bed state bed fining and paving and bed load observations allowed us to conclude that bed load sheets resulted from very efficient vertical and longitudinal grain sorting that produced periodic local bed aggradation and erosion clearly observed in the upstream section of the flume fractional transport rates were measured in one run combined with the results of experiments previously conducted by authors with uniform sediments this experiment showed that the highest peak solid discharges were essentially caused by the much greater mobility of the coarser gravels when transported within bed load sheets a scenario is proposed for the mechanisms involved in bed load sheet production and migration

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

predicting incipient motion including the effect of turbulent pressure fluctuations in the bed 1 incipient motion of particles on a river bed can in principle be understood and predicted from a balance of the forces acting on the particles on a natural river bed the exposure of particles to the flow is variable the effect of exposure on the initiation of motion including the case of deep embedding is studied in this paper so far the turbulence induced lifting force has been derived exclusively from examinations of the surface flow the understanding of destabilization processes of the riverbed is improved when turbulence induced vertical pressure gradients in the upper sediment layer are included this is particularly important for particles that lie in the lee of larger ones an analytical solution for the critical shields parameter of spherical particles is found including the effects of pressure fluctuations in the bed macroscale flow structures average bed slope and shallow flow existing laboratory data designed to test the effect of exposure and roughness conditions are in excellent agreement with the new model

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

the last 50 000 years in the neotropics southern brazil evolution of vegetation and climate abstract in the lagoa campestre lake of salitre 19 s 46 46 w 970 m elev there are plant taxa belonging to many ecological groups that are encountered nowhere else at this latitude frequent incursions of polar advections causing cooling and humidity a cool and foggy climate in the middle of the depression and warmer temperatures on the surrounding slopes help to maintain all these groups within a fairly restricted area late pleistocene holocene climatic change has had a considerable impact on the flora and vegetation of salitre the pollen record of the 6 m deep core lc3 shows how cold forest trees such as araucaria angustifolia and drimys brasiliensis semi deciduous forest halophytic plants and peat bog started to develop on this site the initial period between c 50 000 and 40 000 yr b p was an arid phase not recorded in any other neotropical lowland site it was followed by a period of high moisture levels 40 000 to 27 000 yr b p with a maximum estimated at c 35 000 yr b p the late glacial maximum is missing because of a gap in sedimentation humidity gradually increased during the late pleistocene between 16 000 and 11 000 yr b p the early holocene 9500 to 5000 yr b p is characterized by a more marked seasonal pattern and higher temperatures reaching a maximum c 5000 yr b p the spread of semi deciduous forest between 4000 and 3000 yr b p attests to a return of humidity comparison with the serra negra section 19 s 46 45 46 w 1170 m elev not far from salitre confirms the high moisture rates recorded at c 35 40 000 yr b p although temperatures were cooler at the altitude of serra negra as is attested by the presence of araucaria forest and also confirms the strong impact of polar advections on the climate of southeastern brazil

fluvial palaeoenvironments in the intracratonic amazonas basin early miocene early middle miocene colombia abstract based on a detailed study of early miocene to early middle miocene sediments in the caqueta river area amazonas basin colombia a reconstruction of the palaeoenvironmental and palaeogeographical history is made sediment composition sedimentary sequences and transport directions indicate that the succession was deposited by a low sinuosity fluvial system with an anastomosing character that originated from the guyana shield basin subsidence precambrian and palaeozoic palaeorelief and a changing base level controlled deposition in this fluvial system sedimentary environments such as channels abandoned channels crevasse splays backswamps and palaeosoils characterized this system where fluvial flooding was common the early miocene to early middle miocene sequences have been truncated by a more recent fluvial system probably of pliocene to pleistocene age representing the precursor of the present caqueta river palynological analysis suggest that the sediments are part of the retitricolporites and the psiladiporites crototricolpites pollen zones fifteen new species are described which belong to the genera psilatricolpites retitricolpites retitricolporites bombacacidites syncolporites and psilastephanocolporites the palynological assemblages indicate that the area was characterized by palm swamps riparian vegetation and a rather diverse tropical forest intervals rich in marine palynomorphs accompanied by high concentrations of mangrove pollen grains zonocostites suggest periods of marine influences and mangrove development it is thought that the marine intervals are related to the late burdigalian transgression

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

a consideration of the dune antidune transition in fine gravel hydraulic data defining the dune antidune transition in fine gravel are compared with potential flow theory and information is drawn from published experiments and field based studies attention is given to both transitional bedforms and the development of downstream migrating antidunes in the latter case most data pertain to sand beds and not to gravel empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower froude numbers at greater relative depths although a critical froude number of 0 84 may reasonably be applied for the beginning of the dune to antidune transformation lag effects and a possible depth limitation ensure that transitional bedforms may persist across a broad range of froude numbers from 0 5 to 1 8 this latter observation has great relevance for palaeohydraulic estimates derived from outcrop data whereas the application of theoretical bedform existence fields based upon potential flow theory to fine gravel was previously purely speculative the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems for the first time laboratory data pertaining to downstream migrating gravel antidunes are compared with theory these bedforms have been reported from certain experimental near critical flows above sand or gravel beds but have been observed infrequently in natural streams however there are no detailed studies from natural rivers and only a few contentious identifications from outcrops nevertheless the limited hydraulic data conform to theoretical expectations

tropical climates at the last glacial maximum a new synthesis of terrestrial palaeoclimate data i vegetation lake levels and geochemistry palaeodata in synthesis form are needed as benchmarks for the palaeoclimate modelling intercomparison project pmip advances since the last synthesis of terrestrial palaeodata from the last glacial maximum lgm call for a new evaluation especially of data from the tropics here pollen plant macrofossil lake level noble gas from groundwater and delta 18 o from speleothems data are compiled for 18 2 ka c 14 32 degrees n 33 degrees s the reliability of the data was evaluated using explicit criteria and some types of data were re analysed using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the coldest month mtco mean annual temperature mat plant available moisture pam and runoff p e cold month temperature mat anomalies from plant data range from 1 to 2 k near sea level in indonesia and the s pacific through 6 to 8 k at many high elevation sites to 8 to 15k in s china and the se usa mat anomalies from groundwater or speleothems seem more uniform 4 to 6 k but the data are as yet sparse a clear divergence between mat and cold month estimates from the same region is seen only in the se usa where cold air advection is expected to have enhanced cooling in winter regression of all cold month anomalies against site elevation yielded an estimated average cooling of 2 5 to 3 k at modern sea level increasing to approximate to 6 k by 3000m how ever neotropical sites showed larger than the average sea level cooling 5 to 6 k and a non significant elevation effect whereas w and s pacific sites showed much less sea level cooling 1 k and a stronger elevation effect these findings support the inference that tropical sea surface temperatures ssts were lower than the climap estimates but they limit the plausible average tropical sea surface cooling and they support the existence of climap like geographic patterns in sst anomalies trends of pam and lake levels indicate wet lgm conditions in the w usa and at the highest elevations with generally dry conditions elsewhere these results suggest a colder than present ocean surface producing a weaker hydrological cycle more arid continents and arguably steeper than present terrestrial lapse rates such linkages are supported by recent observations on freezing level height and tropical ssts moreover simulations of greenhouse and lgm climates point to several possible feedback processes by which low level temperature anomalies might be amplified aloft

late quaternary pollen records from the middle caqueta river basin in central colombian amazon abstract three pollen records are presented from the pantano de monica region 0 42 s 72 04 w 160 m elevation on the lower terrace of rio caqueta of the central colombian amazonas ten radiocarbon dates from three cores indicate that the deposits are of holocene age but the pollen data suggest that the record may also contain the late glacial the core pantano de monica 1 covers the time interval from 11 150 bp extrapolated to 4730 bp during the late glacial and early holocene this swamp was smaller in size and waters were more shallow than today with abundant mauritia palm trees this indicates that the lower terrace of the caqueta river was better drained than today which might be related to changes in the drainage system and or drier conditions during that time late glacial and early holocene vegetational changes in the rain forests surrounding the swamp pantano de monica indicate successional stages probably related to changes in the drainage system and or climate changes presence of podocarpus pollen grains up to 2 6 of the total sum and influx of 78 grains cm 2 yr 1 point to the regional presence of podocarpus at the beginning of the holocene evidence of podocarpus during glacial times in other pollen records from the amazon basin has been taken as indicative of cooling core pantano de monica 2 was taken in a small swamp 1000 m distance to pantano de monica 1 and starts at 4000 bp when this location was well drained several changes in the composition of the rain forest are documented such as increase in disturbance increase of cecropia followed by a quick recovery increase of psychotria protium and caryocar tree taxa were frequent suggesting that the lower terrace area was well drained until 3080 bp after which small swamps developed core pantano de monica 3 is from the wet forest in between the two swamps and represents the period since 3260 bp pollen data show a continuously increasing presence of palms probably euterpe suggesting either a transition from well drained to less drained conditions in the lower terrace during the late holocene and or a more intensive human impact of the rain forest on the lower terraces of rio caqueta the three records from the lower terrace of rio caqueta show different forest compositions in the past and indicate that the rain forest environments were not stable during late glacial and holocene times comparison with other data from lowland records of northwestern south america suggest that climate change is a major factor for environmental change in central colombian amazon

bed load transport flume experiments on steep slopes experiments were conducted over uniform gravel bed materials to obtain 143 friction factor values under bed load equilibrium flow conditions in an attempt to add to the scarce data available on slopes between 1 and 9 for shields numbers between 0 08 and 0 29 analyses showed that when only flows over flat beds are considered a distinction must be made between flows with and without bed load more particularly fitting flow resistance equations indicated that the roughness parameter increases by a factor of 2 5 from clear water flow to intense bed load transport between these two states the flow resistance can be approximated by a constant for a given slope

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

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

observations of nearshore circulation rip currents a sector scanning doppler sonar was deployed at the end of the scripps institution of oceanography pier aimed in toward the surf zone estimates of acoustic backscatter intensity and radial velocity were obtained seaward of the break point over a wedge with an arc of about 45 and a radius of 200 to 400 m the acoustic intensity is proportional to some combination of the bubble density and the suspended sediment in the water well defined jets of water extending seaward from the surf zone or rip currents were observed these rip currents occurred episodically recurred aperiodically one to four times per hour or so and lasted several minutes each the water transported offshore within the rips was rich in suspended sediment and bubbles producing distinctive bright patterns that can be easily recognized in successive images of backscatter intensity from the sonar fluid velocities were estimated from the doppler shift of the sonar signal approaching 0 7 m s in some of the events simultaneously a transect of temperature density and turbidity within the rip current was obtained with conductivity temperature depth ctd and optical backscatter strength obs sensors these data describe the ejection of warm turbid water from the surf zone and provide a comparison between the acoustic intensity and the optical turbidity of the fluid the horizontal extent of the sonar measurements and the vertical resolution of the ctd obs measurements complement each other in the study of the dynamics of the nearshore environment exchange rates are not fully resolved here since only one seaward jet is contained within the field of view however these observations suggest that rip currents could cause significant exchange between the nearshore and offshore waters

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

finite amplitude river dunes the linear and weakly nonlinear stability of a uniform flow in an infinitely wide open channel with erodible bottom is studied under suitable conditions the flow is found to be unstable leading to the formation of dunes and antidunes at a linear level the corresponding regions of existence are presented and compared with experimental data a weakly nonlinear analysis is then performed in a neighbourhood of the critical conditions for dune and antidune formation the analysis shows that for values of the ratio of the shear velocity to the depth averaged velocity of practical interest dune bifurcation is supercritical whereas antidune bifurcation is subcritical the latter result suggests a possible interpretation of the plane antidune transition where plane bed and antidune configurations are observed to coexist for the same values of the flow and the sediment parameters the supercritical behaviour of the dune bifurcation allows for the prediction of an equilibrium amplitude that successfully compares with the amplitudes measured in laboratory experiments

antidunes on steep slopes 1 when rates of subcritical flow are increased on gentle slopes the bed successively produces ripples dunes and flat beds antidunes defined here as all bed undulations for which the surface gravity waves are in phase with the bed profile appear only in high flow rates and may be found in some extreme natural flow events inversely on steep slopes flume experiment s 1 approximately flows are supercritical and antidunes are observed to appear just after the beginning of sediment motion and to disappear for high flow rates using data from new experiments we seek to improve the prediction of antidune geometry on steep slopes an equation for antidune wavelength was deduced from dimensional analysis and fitted to new experimental data the equation was successfully evaluated using a data set extended to 167 values with data from the literature obtained on both steep and gentle slopes this equation gives results very similar to the usual analytical equation from kennedy when tested on the results from gentle slope experiments but proved to be better adapted for antidune wavelength on steep slopes

paleohydrology and paleoclimate of the past 33 000 years at the tamandua river central brazil abstract deposits of the tamandua river contain evidence for four major paleohydrologic stages in the last 33 000 years a wet period between 33 000 and 20 000 14 c yr b p produced a high water table that allowed organic rich deposition in the tamandua valley a dry interval 17 000 10 000 14 c yr b p produced sandy deposits of braided channels and alluvial fans river aggradation during this period probably resulted from a high sediment load promoted by intense slope erosion and from flash floods a wet period after 10 000 and before 6000 14 c yr b p was marked by reduced slope erosion and by high discharge that led to erosion of the valley fill during that time forests developed widely in brazil a drier climate after 6000 14 c yr b p caused a reduction of discharge but allowed a high water table to be maintained

feedback between bed load transport and flow resistance in gravel and cobble bed rivers 1 to calculate bed load engineers often use flow resistance equations that provide estimates of bed shear stress in these equations on the basis of the estimate of the appropriate hydraulic radius associated with the bed only the bed roughness ks is commonly set as a constant whatever the bed load intensity however several studies have confirmed the existence of feedback mechanisms between flow resistance and bed load suggesting that a flow dependent bed roughness should be used therefore using a data set composed of 2282 flume and field experimental values this study investigated the importance of these feedback effects new flow resistance equations were proposed for three flow domains domain 1 corresponds to no bed load and a constant bed roughness ks d where d is a representative grain diameter whereas domain 3 corresponds to a high bed load transport rate over a flat bed with a constant bed roughness ks 2 6d between these two domains a transitional domain 2 was identified for which the bed roughness evolved from d to 2 6d with increasing flow conditions in this domain the darcy weisbach resistance coefficient f can be approximated using a constant for a given slope the results using this new flow resistance equation proved to be more accurate than those using equations obtained from simple fittings of logarithmic laws to mean values the data set indicates that distinguishing domains 2 and 3 is still relevant for bed load in particular the data indicate a slope dependence in domain 2 but not in domain 3 a bed load model based on the tractive force concept is proposed finally flow resistance and bed load equations were used together to calculate both shear stress and bed load from the flow discharge the slope and the grain diameter for each run of the data set efficiency tests indicate that new equations implicitly taking a feedback mechanism into account can reduce the error by a factor of 2 when compared to other equations currently in use showing that feedback between flow resistance and bed load can improve field bed load modeling

incipient sediment motion at high slopes in uniform flow condition 1 this paper reviews the problem of the incipient motion of cohesionless particles in a high sloping water stream a theoretical analysis of the equilibrium of a gravel particle is developed by a deterministic approach with the introduction of the relative degree of exposure of the particle itself to the stream flow the parameter relative degree of exposure relates the positioning of a particle with the upstream one the problem of velocity distribution and the definition of drag coefficient is also revised to better account for low submergence the results of the theory are compared with a series of ad hoc experiments performed by c gregoretti

lake levels since about 40 000 years ago at lake chalco near mexico city abstract diatoms magnetic susceptibility organic content and14c ages of sediments from a 26 m core suggest that lake chalco in the southern part of the basin of mexico went through a series of major fluctuations during the late pleistocene and the holocene before ca 39 00014c yr b p the lake was very deep about 8 10 m alkaline and saline it then became shallow

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

late quaternary sediment fluxes from tropical watersheds abstract inherited saprolite stores and continued weathering in quaternary time juxtapose abundant clay and fresh rock in tropical landscapes this influences sediment fluxes and affects the interpretation of sediment sequences derived from tropical watersheds detrital kaolinites derive from inherited saprolite sources as well as from soil clays and appear in delta and ocean sediments these sediments appear to correspond with sub milankovitch millennial scale cycles of climate change but may also record century scale episodes of rapid warming dansgaard oeschger events destabilisation of sediment sources and increased sediment fluxes in the late quaternary followed millennia of climatic deterioration cooling aridity and vegetation change and led to altered patterns of sedimentation during the last glacial maximum lgm sediment yield from slopes increased 10 around the lgm when rainfall was reduced by 30 60 and led to fan building and braided channels rainfall increased 40 80 from the lgm to the early holocene maximum and this led to channel cutting and major sediment fluxes to delta and ocean sinks vegetation recovery lagged the rapid warming by several millennia and was interrupted by younger dryas yd aridity influencing slope and stream behaviour holocene sedimentation has been by both vertical and lateral accretion increasing floodplain sediment stores

dimensions of standing waves at steps in mountain rivers 1 oscillations of the water surface are common features of flows around critical conditions fr 1 and of both natural and structure induced undular hydraulic jumps the paper describes experimental tests and field data on the dimensions of standing waves created by artificial drops in steep gravel bed rivers the purpose of the research is to analyze what parameters affect the phenomenon to provide semiempirical formulations for wave dimensions and to assess whether previous models for undular hydraulic jumps and antidunes are reasonably predictive the dimensionless wavelength and wave amplitude were correlated to the froude number at the sill the first wavelength appears linearly related to the froude parameter whereas the amplitude of the first wave shows a less defined behavior possibly indicating that a maximum value occurs for sill froude numbers around 1 6 1 7 the corresponding wave steepness i e wave amplitude length results between 0 15 0 16 and this range also characterizes the point at which the dimensionless trough depth below the sill achieves a local maximum probably related to hydrodynamic conditions just before wave breaking the transition between roller and undular jumps at drops is then argued to likely represent a critical stage in step pool streams and the hydraulic and morphological implications of this finding are also discussed with particular emphasis on step stability and step wavelength

theoretical development on the effects of changing flow hydraulics on incipient bed load motion 1 several decades of flume and field measurements have indicated that in rough turbulent flows the critical shields stress increases with increasing slope and associated decreasing relative depth this result contradicts the usual consideration of a decreased critical shields value on very steep slopes because of increased gravitational effects however recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds these measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed whose properties mean velocity and turbulence depended on the flow s relative depth unfortunately velocity profile measurements for low relative depth associated with steep slopes are scarce and it is still difficult to include such flow properties in a force balance model flow resistance data on the basis of depth average velocity measurements are very common and cover a wide range of slopes and relative depths in this paper these data are used to fit a velocity profile including a roughness layer when used in a force balance model for incipient motion it adequately reproduced a data set composed of 270 critical shields values measured in a flume with near uniform sediments the relevance of this research to field problems is discussed using a data set composed of 92 critical shields stresses obtained from field measurements finally a model is proposed for field applications taking into account the slope effect

the last glacial maximum stability and change in a western amazonian cloud forest the climatic and vegetation history of a western amazonian cloud forest is documen ted in a continuous pollen record spanning the full last glacial period between 43 5 and 22k cal yr bp the chronology for this record is based on eight radiocarbon dates yielding a low resolu tion pollen analysis for the region a bioclimatic envelope model was generated on the basis of mod ern altitudinal distributions and the pollen data which produced a glacial palaeotemperature estimate of ca 5 c relative to present palaeoecological evidence of continuous moist cloud forest cover in the basin indicates that western amazonian forests were not fragmented during the lgm this evidence supports wet conditions in western amazonia at the lgm and further refutes hypotheses of amazonian aridity during the last ca 44k cal yr bp copyright 2005 john wiley sons ltd

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

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

flume study of the effect of relative depth on the incipient motion of coarse uniform sediments an experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0 3 m wide tilting flume the concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport the experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter a revised shields diagram relating critical stress grain reynolds number and depth to grain size ratio has been derived this has implications for the calculation of sediment transport rates design of stable alluvial channels and physical hydraulic modeling

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

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

step pool evolution in the rio cordon northeastern italy the principle that formative events punctuated by periods of evolution recovery or temporary periods of steady state conditions control the development of the step pool morphology has been applied to the evolution of the rio cordon stream bed the rio cordon is a small catchment 5 km2 within the dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded detailed field surveys of the step pool structures carried out before and after the september 1994 and october 1998 floods have served to illustrate the control on step pool changes by these floods floods were grouped into two categories the first includes ordinary events which are characterized by peak discharges with a return time of one to five years 1 8 5 15 m3 s 1 and by an hourly bedload rate not exceeding 20 m3 h 1 the second refers to exceptional events with a return time of 30 50 years a flood of this latter type occurred on 14 september 1994 with a peak discharge of 10 4 m3 s 1 and average hourly bedload rate of 324 m3 h 1 step pool features were characterized primarily by a steepness parameter c h ls s the evolution of the steepness parameter was measured in the field from 1992 to 1998 the results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events during this period bed armouring dominate the sediment transport response however following an extraordinary flood and unlimited sediment supply conditions the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing the analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed copyright 2001 john wiley sons ltd

forest changes in the amazon basin during the last glacial maximum we report here the discovery of forest beds in the amazon basin radiocarbon dated to the wisconsin wurm glaciation these are the first dates from the last glaciation determined on amazonian samples the locality is in ecuador at 1 100 m elevation near the upper limit of the present amazonian rainforest both pollen spectra and podocarpus wood in the deposits suggest that vegetation comparable to the present andean forest grew at least 700 m lower than it does now suggesting a temperature depression of at least 4 5 c for the amazon lowlands

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

bed morphology and generation of step pool channels flume experiments have been carried out to study the formation processes and the bed morphology of step pool channels from the experiments different step types and step configurations could be distinguished depending on the stream power these step types can be seen as an image of the generation mechanisms of step pool systems these results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution in contrast to a variety of other studies antidunes did not contribute to the generation of the step structures however the data of the presented study fits well into the region of antidune formation proposed by kennedy for sand bed rivers this observation points out that step pool field data located in the kennedy region do not inevitably prove that antidunes played a role in step development it is rather proposed that in kennedy s region of antidune formation there exist hydraulic conditions where the flow resistance is maximized it is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height independently of whether these are antidunes in sand and gravel bed rivers or step pool units in boulder bed streams the considerations of the kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self stabilization by modifying the step pool structure towards a geometry that provides maximum flow resistance and maximum bed stability copyright 2008 john wiley sons ltd

holocene history of the choco rain forest from laguna piusbi southern pacific lowlands of colombia abstract a high resolution pollen record from a 5 m long sediment core from the closed lake basin laguna piusbi in the southern colombian pacific lowlands of choco dated by 11 ams14c dates that range from ca 7670 to 22014c yr b p represents the first holocene record from the choco rain forest area the interval between 7600 and 610014c yr b p 500 265 cm composed of sandy clays that accumulated during the initial phase of lake formation is almost barren of pollen fungal spores and the presence of herbs and disturbance taxa suggest the basin was at least temporarily inundated and the vegetation was open the closed lake basin might have formed during an earthquake probably about 440014c yr b p from the interval of about 600014c yr b p onwards 200 different pollen and spore types were identified in the core illustrating a diverse floristic composition of the local rain forest main taxa are moraceae urticaceae cecropia melastomataceae combretaceae acalypha alchornea fabaceae mimosa piper protium sloanea euterpe geonoma socratea andwettinia little change took place during that time interval compared to the pollen records from the rain forests of the colombian amazon basin and adjacent savannas the choco rain forest ecosystem has been very stable during the late holocene paleoindians probably lived there at least since 346014c yr b p evidence of agricultural activity shown by cultivation ofzea maissurrounding the lake spans the last 1710 yr past and present very moist climate and little human influence are important factors in maintaining the stable ecosystem and high biodiversity of the choco rain forest

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

critical shear stress for incipient motion of sand gravel streambeds results of an experimental study of the incipient motion of streambeds composed of sand gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments the experiments were conducted in an 8 m long by 0 30 m wide glass walled tilting flume and an 18 m long by 0 80 1 10 m wide trapezoidal concrete channel a reference transport method is used to define the beginning of bed material movement the experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size intergranular effects mixture standard deviation effect of the shape of grain size distribution absolute value of median size absolute size effect and bed slope effect of relative depth on overall flow resistance the shear stress at incipient motion of median sized grains in mixtures is found to be the same as for uniform sediment of this size the present findings are consistent with available flume and field data a technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal weakly bimodal sediment mixtures is proposed

vegetation development in an amazonian peatland recent investigations of wetlands in western amazonia have revealed the presence of extensive peat deposits up to 7 5 m thick developing under a variety of vegetation types we report the first attempt to establish the long term centennial to millennial scale vegetation history of one of these peatland sites quistococha a palm swamp close to iquitos in northern peru pollen and sedimentological analyses show that peat formation began at the core site under sedge fen or floating mat vegetation c 2200 calendar years before present cal yr bp a seasonally flooded woodland developed c 1880 cal yr bp the permanently waterlogged palm swamp which persists today began to form c 1000 cal yr bp with the present vegetation community established by c 400 years ago the vegetation at this site has undergone continuous change throughout the period of peat formation with several abrupt transitions and reversals and repetitions in the apparent trajectory of change the pollen data combined with sedimentary evidence suggest that the dominant control on ecosystem functioning and development is the flooding regime there appears to have been a decrease in fluvial influence over time there is no clear evidence of direct climatic or anthropogenic influence although we cannot rule out the possibility of climatically driven hydrological changes our results caution against adopting a simple model of peatland vegetation succession in this region

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

palaeoenvironmental dynamics and underlying climatic changes in southeast amazonia serra sul dos carajas brazil during the late pleistocene and holocene abstract pollen fern spore micro charcoal sediment and mineral analyses have been carried out on a 466 cm long sediment core from the serra sul dos carajas swamp in southeastern amazonia ten ams radiocarbon dates suggest that the sediment covers at least the last 73 000 years shifts between tropical forests and savanna changes in species composition as well as changes in fire frequency indicate several wet and dry periods during this time interval during the assumed early pleniglacial 73 50 ka the dominance of savanna vegetation reflects drier conditions whereas vegetation changes in the mid pleniglacial 50 40 ka suggest alternating wet and dry phases under generally cool conditions the recorded next period before the last glacial maximum lgm shows a transition to wetter and cooler conditions followed by a cold and dry lgm and late glacial at the beginning of the holocene between 11 4 and 10 2 cal ka tropical forest expanded reflecting stable and very humid conditions during the early to mid holocene an increasingly seasonal generally warm and dry climate established dry seasons were longer than at present during mid holocene times with a pronounced dry phase from 9 to 3 4 cal ka only after 3 4 cal ka did modern tropical rainforests became established under present day climatic conditions comparison with other palaeoecological studies in the amazon basin and eastern andean regions suggests that environmental changes in serra sul were strongly coupled with large scale climatic changes and sea surface temperature variability in the tropical atlantic

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

step pool formation models and associated step spacing step pool bedforms develop under conditions of high flow rates and high sediment influx and transport rates step setting events are capable of mobilizing and rearranging the large step forming grain sizes thus observations of the processes of step formation are limited to flume experiments the results of laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps are presented over 350 step formations were observed and documented creating a large data set for detailing and evaluating step formation processes existing step formation models focus on step development from antidunes particle clusters transverse ribs or depositional berms or from a hydraulic regime that includes cascade flow and hydraulic jumps these models were developed using field measurements of artificial grade control structures or stable step pool systems during low flow conditions or degradational flume experiments where flows elucidated the step pool bedform in the channel profile but did not create equilibrium transport of the large step forming clasts this research evaluates the pre existing formation models and proposes three new models the rough bed exhumation and dune models the rough bed model was dominant throughout the experiments followed by the exhumation model and then the dune model a single model was not responsible for all step formations the rough bed exhumation and dune models all influenced step development and every step sequence was created through a mixture of formation processes this research generated a large data set where each step spacing is paired with the corresponding step formation model making it possible to identify a distinct exclusion zone length for each model thus given knowledge of the step formation model the minimum distance to the next step downstream can be predicted from the final spacing of a step sequence it may be possible to deduce the likely formation mechanisms active during a step resetting event copyright 2007 john wiley sons ltd

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

tropical south america during the last glacial maximum evidence from glacial periglacial and fluvial records abstract the records of the lgm climates of tropical south america are still sparse compared to the vast region under consideration nevertheless a number of records hold up to critical evaluation early attempts to reconstruct the lgm temperature and precipitation conditions for the lgm around 21 14 c ka bp have suggested that relative to present lowland temperatures were on average only a few c cooler whereas in the andes the temperature depression was in the range of 6 8 c these reconstructions further suggested that tropical precipitation was more limited in both the lowlands and in the andes although recent pollen evidence supports significantly cooler tropical lowland temperatures during the lgm additional alkenone evidence from sea surface temperatures and model analysis are contradictory the data from tropical south america suggest that the late quaternary climatic changes of the lowlands as well as of the andean mountains reflect a response 1 to environmental changes in the source area of the moisture and 2 to global temperature fluctuations based on glacial periglacial and pollen evidence a lgm temperature depression of 5 6 c can be assumed for both tropical lowland south america and the andes these lgm values for the cooling correspond with recent coral records from barbados and the southwestern pacific ice core records from peru noble gas measurements in brazil and ocean core records from the western equatorial atlantic no difference in the value of lgm cooling can be recognised between the tropical lowlands and the high mountains of south america furthermore in all regions the lgm climates were more arid than the present day climates terrestrial records have not yet yielded records of younger dryas age apart from the tropical andes of ecuador where the younger dryas time is characterised by a cooler and drier climate pollen evidence for cooler younger dryas conditions comes from the bolivian andes

neotectonic influence on tropical rivers of southwestern amazon during the late quaternary the moa and ipixuna river basins brazil abstract neotectonic influence has conditioned the development of the moa and ipixuna fluvial belts in the southwestern brazilian amazon region the area underwent a compressive tectonics with reverse n s fault dipping to the west that formed an en echelon system system from west to east from the serras do divisor ranges to the jurua river as a response to stress subsiding blocks with nw se trends were developed the e w to n55 w topographic trends could represent transcurrent dextral transtensional faults width of alluvial belts increases along the sequence of en echelon system blocks correlated with the increase of width the quantity of channels formed by avulsion increases as well the moa and ipixuna alluvial belts respectively reaching ca 9 and 7 km maximum width are abnormally wide because they developed in subsiding blocks stretched in a nw se trend fluvial dynamics was dominated by avulsion in the subsiding blocks during the late pleistocene and probably during part of the holocene

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

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

geomorphic response to rapid climatic and hydrologic change during the late pleistocene and early holocene in the humid and sub humid tropics abstract the data base concerning late quaternary environmental change in the humid tropics is dependent on records from scattered sites the intensity and duration of wet dry oscillations remain speculative and the responses of hillslopes and river systems are based on over simplified models however available evidence indicates that prolonged aridity affected all but a few favoured core areas of equatorial climate after 20 000 bp lasting 5 7 ka dry conditions at the lgm were marked by semi arid landforms and reduced steam activity large palaeofloods occurred after 13 000 be but dry conditions returned after 11 000 bp before the early holocene pluvial led to abundant sedimentation lasting nearly 2 ka after 9500 bp in africa amazonia and australasia erosion channel cutting and flood deposition occurred at the pleistocene holocene transition followed by multiple shifts from lateral to vertical accretion fill cut episodes during the early holocene while thick overbank deposits formed during the later holocene re establishment of the lowland rainforests was delayed until after 9000 bp in africa australia and brazil and several wet dry oscillations followed in the mid holocene period hillslope activity at the lgm was marked by local sediment transfers and fan formation at the termination sediment fluxes increased rapidly but mass movement probably peaked after 10 000 bp sediment delivery to stream channels was not immediate and large sediment stores remain on the landscape today many in areas of potential sensitivity of erosion

regional patterns of well drained upland soil differentiation in the middle caqueta basin of colombian amazonia abstract as part of a land ecological survey of the middle caqueta basin in colombian amazonia a total of 41 well drained upland soil profiles under primary rain forest was studied two soil groups were recognised on the basis of principal components analysis pca soils pertaining to the first group cover about 45 60 of the well drained uplands and classify as afsols and acrisols they are characterised by fine textured reddish b horizons and are low in exchangeable bases but high in exchangeable al furthermore they contain relatively high total nutrient reserves soils pertaining to the second group cover about 35 50 of the well drained uplands and classify as acrisols and ferralsols these soils are characterised by medium textured yellowish b horizons within 100 cm depth are low in exchangeable bases and al and show very low total nutrient reserves the soils of the ali acrisol group are mostly developed in andean origin deposits of the low terraces of the caqueta river and the tertiary pebas formation the soils of the acri ferralsol group are mostly developed on shield derived medium to coarse textured tertiary deposits and are also found on palaeozoic sandstone outcrops the differences between the soils of the two groups are therefore principally related to variation in the parent materials and reflect the principal geological structure of the middle caqueta area a similar kind of well drained upland soil differentiation is found in the colombian amazon area as a whole including footslope areas near the andes and probably also in adjacent parts of peru and brazil the presence of the soils of the acri ferralsol group in the middle caqueta area and in nw amazonia is in conflict with general statements that well drained upland soils in upper amazonia would be less weathered than those from central or eastern amazonia the reddish or yellowish colour of the b horizon within a depth of 100 cm provides a practical field characteristic to distinguish the two groups of soils the colour of the b horizons seems mainly related to dynamics of the soil organic matter in sandy profiles of the soils from the acri ferralsol group a slight degree of podzolisation may occur in the topsoil current international soil classification systems do not adequately separate the two well drained upland soil groups in the middle caqueta area beacuse total nutrient reserves are not sufficiently incorporated and too much emphasis is laind on th presence of a clay increase

holocene flooding and species diversity in southwestern amazonia abstract a thick blanket of holocene alluvium lies over southwestern lowland amazonia and may possibly occur throughout much of the amazon basin these deposits resulted from massive seasonal flooding from about 11 000 to about 5000 yr b p that was followed by two cycles of erosion and deposition interpretations based on these geologic data suggest that southwestern lowland amazonia is ecologically an island in a state of supersaturated disequilibrium as a result of colonization from pleistocene refugia on its perimeter and that habitats of highest diversity may be the most recent in origin conservation efforts and studies of amazonian biogeography soils and paleoethnography should be reevaluated in light of the geologic data

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

understanding the impacts of late quaternary climate change in tropical and sub tropical regions abstract the understanding of climate impacts on tropical rivers and catchments has developed in part from models developed for temperate landscapes and a common rhythm is often apparent when millennial scale studies are compared at this level issues arising from the complex response of rivers to internal and external factors appear less important than the climate driven signal understanding the fluvial record however depends on the availability and interpretation of other sources of proxy data such as pollen spectra lake levels and ocean cores in many rainforest areas few pollen records exist and many of these reveal a hiatus at the time of the last glacial maximum examples of fluvial archives from africa indonesia and tropical queensland australia are discussed in relation to independent records of vegetation change and inferences are made concerning critical episodes in fluvial history focusing on the transitions from oxygen isotope stage ois 3 towards glacial conditions in ois2 and the pleistocene holocene transition to modern climates links between climate vegetation change and fluvial activity are presented as descriptive models

the response of geomorphic systems to climatic and hydrological change during the late glacial and early holocene in the humid and sub humid tropics

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

late quaternary stream sedimentation in the humid tropics a review with new data from ne queensland australia there is now a wide agreement that temperature depression in the humid tropics during the last glacial maximum lgm was at least 5 c most estimates of precipitation reduction at the lgm range from 25 30 to 50 65 based on proxy data but the recent ccm1 model envisages only around 12 dates obtained from river sediments indicate major changes to fluvial activity in the late quaternary isotope zone 3 sediments 58 28 ka bp are widespread and possibly indicate cooler conditions post 28 ka bp and certainly post 21 ka bp river regimes altered radically towards fan building braiding or major reduction in all activity this paper reports on fan formation in ne queensland between 26 and 14 ka bp and reviews evidence for comparable changes in humid tropical areas of s america w africa and se asia including records of holocene sedimentation within a global rhythm of major changes to river regimes in the humid tropics during the late quaternary it is now possible to detect regional variations in stream response to climatic change at the lgm reductions in stream power may have led to fan formation in ne queensland while vegetation changes may have contributed to increased sediment loads and braiding in some forest marginal areas but in w africa greater aridity may have been responsible for enfeebled streams leaving few records channel cutting then deposition of coarse sediment in braided rivers marked the transition to the early holocene in w africa and fans became entrenched in ne queensland this regime persisted until forest recovery was complete by 9 5 8 5 ka bp when widespread overbank deposition occurred and a change towards meandering channels took place widely across the humid tropical zone followed by several cut and fill episodes in the middle and late holocene

variation in the reference shields stress for bed load transport in gravel bed streams and rivers 1 the present study examines variations in the reference shear stress for bed load transport r using coupled measurements of flow and bed load transport in 45 gravel bed streams and rivers the study streams encompass a wide range in bank full discharge 1 2600 m3 s average channel gradient 0 0003 0 05 and median surface grain size 0 027 0 21 m a bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate w versus the reach average dimensionless shear stress the reference dimensionless shear stress r was then estimated by selecting the value of corresponding to a reference transport rate of w 0 002 the results indicate that the discharge corresponding to r averages 67 of the bank full discharge with the variation independent of reach scale morphologic and sediment properties however values of r increase systematically with average channel gradient ranging from 0 025 0 035 at sites with slopes of 0 001 0 006 to values greater than 0 10 at sites with slopes greater than 0 02 a corresponding relation for the bank full dimensionless shear stress bf formulated with data from 159 sites in north america and england mirrors the relation between r and channel gradient suggesting that the bank full channel geometry of gravel and cobble bedded streams is adjusted to a relatively constant excess shear stress bf r across a wide range of slopes

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

late glacial and holocene sedimentation and fluctuations of river water level in the caqueta river area colombian amazonia the sequence of late glacial and holocene alluvial sedimentation in the middle caqueta river basin of colombian amazonia is described based on the study of the sediments and palynology of several river bank sections and on 30 radiocarbon dates an early late glacial sedimentation cycle is recognised followed by a minor late late glacial erosion phase the holocene valley fill consists of grey clays often present in the lower part of the sections deposited in open water and silty clays often with faint yellow mottling deposited under a regime of seasonal flooding the base of the holocene sections is formed by sands where exposed in two places the transition of sand to open water grey clay was dated around 10 000 yr bp and there is a suggestion that open water may have been more common at the beginning of the holocene than later when sedimentation by seasonal flooding became important in many places much of the earlier holocene sediments may have been removed by erosion and replaced by younger sediments by a process of lateral aggradation a considerable part of the present valley fill is younger than ca 3500 yr however in several places older holocene sediments are found apparently only little affected by later erosion lying below younger varzea silty clays during the holocene more organic sediments were formed in periods with reduced river discharge related to drier climates in the andes and possibly in amazonia these dry periods deduced from data in the caqueta river area correspond well with dry phases in other parts of northwestern south america e g between approximately 2700 1900 yr bp and approximately 3200 3800 yr bp rates of average net sedimentation calculated from dated sections that apparently lack major hiatuses caused by erosion were high in the lower holocene low during the middle holocene and increase again in the upper holocene levee deposits became coarser and the high river level of the caqueta increased during the late holocene these phenomena may be explained by the increasing influence of man on the vegetation cover in the andean headwater areas and possibly also in the amazonian catchment area of the caqueta river

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