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

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

late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

late quaternary paleosols and their paleoenvironmental significance along the andean piedmont eastern bolivia the andean piedmont of eastern bolivia is situated at the southern margin of amazonia characterized by an overall humid climate regime with a marked contrast between the rainy and dry seasons the nearby subandean foothills deliver abundant sandy sediments to the piedmont leading to a complex array of sediments and paleosol horizons within this setting the presented study analyzes four profiles of paleosol sediment sequences along incised ephemeral streams near santa cruz de la sierra with a focus on past pedogenic variability in the context of the regional late quaternary geomorphic and environmental evolution based on field observations micromorphological analysis geochemical and clay mineralogical data five classes of paleosol horizons could be distinguished the individual paleosol horizons as well as the sediments in which they developed were interpreted regarding their paleoenvironmental significance taking into consideration the various controls on soil formation with particular focus on changes of local environmental conditions through time thus three different pathways of soil formation were established on the late quaternary timescale the results suggest a strong relation between paleoenvironmental conditions climate vegetation etc soil environment soil water flow micro environment and the type of paleosol horizons developed in the study area the formation of red beds bw horizons implies very dry soil environments under dominantly dry conditions which seem to have prevailed in the study area some time before 18 cal ka bp moderately dry but markedly seasonal environmental conditions with a long dry season and strong seasonal contrasts in soil water flow could explain the formation of moderately developed bwt horizons around 18 cal ka bp and much of the mid holocene the formation of bt horizons and or clay lamellae in relation to intense neoformation of clay and dominant clay illuviation by soil water points to wet conditions similar to today which have probably prevailed in the study area before 8 cal ka bp and since 5 cal ka bp

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

an environmental reconstruction of the palaeo amazon river system middle late miocene nw amazonia abstract new sedimentological and palynological data from the tertiary sediments in the upper amazon river area suggest that these sediments are fluvio lacustrine deposits of middle to late miocene age they were generated as a result of the uplift of the eastern cordillera andes and constitute possibly the oldest relics of the amazon river system the palaeoenvironment in which these sediments were deposited is characterized by extensive wetlands environments formed by swamps shallow lakes crevasse splay channels and crevasse delta lakes where the channel environment is poorly represented the palaeovegetation was dominated by palms e g mauritia and grimsdalea riverine taxa e g bombacaceae amanoa and alchornea ferns and fern allies e g polypodiaceae and selaginellaceae floating meadows gramineae and aquatic taxa ceratopteris botryococcus and azolla the relative abundance of gramineae and the occurrence of andean type pollen taxa is related to the andean origin of the fluvial system the varzeas of the present upper amazon river flood basin are probably the best analogue for the middle to late miocene environment intervals rich in marine palynomorphs mangrove pollen brackish tolerant molluscs and ostracods and ichnofossils of the thalassinoides teichichnus association suggest that the palaeoenvironment was characterized by brackish conditions and marine influence these marine incursions are possibly related to the langhian and the serravallian global sea level rise although in the middle miocene a global cooling is known to have occurred no indicators of a cooler climate have been observed in the miocene palynoflora of the upper amazon river area finally four new sporomorph species are described belonging to the form genera psilatriletes clavainaperturites and psilaperiporites

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

preliminary results from u th dating of glacial interglacial deposition cycles in a silica speleothem from venezuela abstract recent explorations in cueva charles brewer a large cave in a sandstone tepui se venezuela have revealed silica biospeleothems of unprecedented size and diversity study of one a sub spherical mass of opaline silica reveals a complex laminated internal structure consisting of three narrow dark bands alternating with two wider light bands uranium thorium dating has produced 3 stratigraphically correct dates on the light bands from 298 6 mis 9 to 390 33 ka mis 11 u concentration is only 30 110 ppb initial 234u 238u ratios are high and increase over time from 1 8 to 5 3 growth rate is very low the fastest at 0 37 0 23 mm ka in mis 9 trace element and heavy metal content of the dark bands is distinctly higher than that of the light bands it is hypothesized that the dark and light bands correlate with drier glacial and wetter interglacial periods respectively and that this sample probably began to grow in mis 13 the cave is in a region that straddles a regionally important ecotone the speleothem isotopic and trace element variations may preserve a useful paleoclimatic signal this is the first published suite of u th dates from a single silica speleothem and the longest quaternary record for this region

holocene landscape evolution along the andean piedmont bolivian chaco abstract as part of the gran chaco the andean piedmont in eastern bolivia is characterized by a highly seasonal climate and large rainfall variability it receives sediments from small drainage basins in the subandean ranges therefore it should constitute a suitable sedimentary and geomorphic archive for the detection of past environmental changes this study reconstructs holocene landscape evolution along the andean piedmont in the bolivian chaco by combining large scale geomorphological analysis and the investigation of sedimentary sequences fluvial aggradation and channel avulsions are the dominant processes of piedmont construction and were particularly pronounced during the mid holocene this is interpreted to reflect a generally active landscape characterized by reduced forest cover as result of increased aridity widespread soil formation and landscape stability in all investigated catchments start after 4 cal ka bp indicating the onset of wetter conditions and successively denser forest cover in the bolivian chaco renewed sedimentation during the last millennium could imply a shift to more arid environmental conditions probably in relation to larger scale climatic phenomena such as the little ice age lia particularly during the last centuries the fluvial system of the study area has undergone the most profound changes of the entire holocene with sedimentation of very coarse sediments and subsequent incision this reflects important changes both in discharge and sediment supply and might be the result of either shorter term climate changes or increasing human impact

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

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

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

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

ichnology and sedimentology reveal depositional characteristics of bay margin parasequences in the miocene amazonian foreland basin abstract proposed depositional models for miocene amazon foreland basin strata pebas formation peru are controversial recent depositional models include lacustrine and tidally influenced brackish water embayment this paper presents data that support tidally influenced brackish water deposition for at least part of pebas time 10 14 ma two parasequences are presented santa julia and tamshiyacu both crop out along the amazon river in upper amazonia near iquitos peru at these locations abundant evidence of brackish water tidally influenced deposition is documented including marginal marine bioturbation sedimentary couplets semidiurnal couplets preserved in burrows and pinstripe lamination the deposits are locally highly bioturbated at both locations ichnogenera normally associated with marine to brackish water depositional environments are common three normally marginal marine ichnofabrics are reported 1 a chondrites reburrowed planolites ichnofabric resident only in massive appearing muds 2 a scolicia laminites thalassinoides ophiomorpha ichnofabric that is manifested as intensely bioturbated silty sands and 3 a thalassinoides generated ichnofabric that is interpreted to have descended into consolidated substrates and is thus representative of the glossifungites ichnofacies several trace fossils contain laminated infills organized into distinct sedimentary couplets that are best interpreted as resulting from semidiurnal processes six conclusions are arrived at 1 sedimentological and ichnological data consistently indicate that sediment accumulation dominantly occurred in sporadically dysaerobic marine to brackish water under a tidal influence 2 sediment accumulation occurred in bay margin environments that prograded into a shallow quiescent bay 3 a stratified water column is indicated by the ichnofauna 4 low accommodation space repetitive and rapid adjustments of relative sea level shallow wave base and a stratified water column combined to generate an atypical parasequence architecture 5 previously published isotopic data are consistent with sediment accumulation in brackish to marine water and 6 marine incursion into amazonia occurred during the middle miocene

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

numerical models of tectonic deformation at the baltica avalonia transition zone during the paleocene phase of inversion abstract predictions from dynamic modelling of the lithospheric deformation are presented for northern europe where several basins underwent inversion during the late cretaceous and early cenozoic and contemporary uplift and erosion of sediments occurred in order to analyse the evolution of the continental lithosphere the equations for the deformation of a continuum are solved numerically under thin sheet assumption for the lithosphere the most important stress sources are assumed to be the late cretaceous alpine tectonics localized rheological heterogeneities can also affect local deformation and stress patterns present day observations available in the studied region and coming from seismic structural interpretations and stress measurements have been used to constrain the model our modelling results show that lateral variation in lithospheric strength below the basin systems in central europe strongly controls the regional deformation and the stress regime furthermore we have demonstrated that the geometry of the boundary between baltica and avalonia together with different rheological characteristics of the two plates had a crucial role on local crustal deformation and faulting regime resulting in the baltica avalonia transition zone from the s n alpine convergence

montane bias in lowland amazonian peatlands plant assembly on heterogeneous landscapes and potential significance to palynological inference abstract past temperature changes in tropical mountain regions are commonly inferred from vertical elevational shifts of montane indicator taxa in the palynological record however temperature is one of several abiotic factors driving the low elevational limits of species and many montane taxa can occur in warmer lowlands by tracking appropriate habitat types especially highly flooded wetlands in this paper we explore ways in which lowland habitat heterogeneity might introduce error into paleo temperature reconstructions based on field data of seven modern peatland vegetation communities in the southern peruvian amazon 200 masl peat rich substrates are common edaphic transitions in pollen cores and provide detailed records of past vegetation change the data show that indicators of modern peatlands include genera with montane as well as lowland distributions while indicators of surrounding forests on mineral substrates have predominantly lowland distributions based on family level analyses we find that modern peatland vegetation communities have taxonomic compositions appearing to be 389 m to 1557 m mean 1050 391 m above their actual elevations due to a high abundance and number of families with high elevation optima we interpret the relatively higher prevalence of montane elements in modern peatlands as habitat tracking of a conserved montane niche on heterogeneous lowland landscapes we suggest that both high moisture availability and stressful edaphic conditions of peatland habitat may explain the montane bias observed to the extent that fossilization provides a better record of past vegetation that occurred proximate to the site of deposition we suggest that habitat tracking of montane elements may introduce a cool bias in lowland paleo temperature reconstructions based on pollen proxies

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

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

geomorphological evidence of recent tilting in the central amazonia region abstract geomorphometric techniques applied to remote sensing data represent powerful tools as an aid for detecting terrain features produced by recent vertical crustal movements in a variety of landscapes in this work geomorphometric analysis of the central amazonia drainage network using shuttle radar topography mission srtm data was carried out to determine surface water loading and tectonic influence on the development of the uatuma and urubu river basins since the miocene the main objective was to detect geomorphological domains of lateral stream migration and channel sinuosity changes that could indicate recent vertical crustal movement to achieve this the transverse topographic symmetry factor and sinuosity index were applied to analyze the study area drainage network comparison of the geomorphological domains with structural field data and records of seismicity suggests that they are influenced by subsurface fault reactivation which coincides in space with faults mapped in outcrops that have activated since the miocene this interpretation is corroborated by the spatial correspondence between these faults with domain boundaries and shapes by river sinuosity change when crossing these boundaries and by a high concentration of seismic activity along one of these limits the crustal movement caused by hydrological loading can also influence such geomorphological features such is the case of lateral stream migrations of a domain in the opposite sense to the amazon river flow this may be a consequence of the more pronounced expression of hydrological loading in the negro and solimoes river confluence region

late pliocene pleistocene high resolution pollen sequence of colombia an overview of climatic change two long continental pollen records funza i 357 m and funza ii 586 m from the high plain of bogota colombia at 2550 m elevation have been studied palynologically fission track dates on zircon from intercalated volcanic ashes provided a chronological framework and showed the pollen records which correlate with high precision to be continuous over the interval from ca 3 2 ma to ca 27 ka the late pliocene pleistocene history of the montane forests and open alpine paramo vegetation is documented with a temporal resolution of ca 6 5 ka and for the upper 1 1 ma with a resolution of ca 1 2 ka the immigration of the northern hemisphere elements alnus at ca 1 ma and quercus at ca 0 33 ma which travelled along the panamanian landbridge caused significant changes in the composition of the andean montane forests the successive pleistocene glaciations forced the andean vegetation belts to an almost continuous altitudinal movement the upper forest line at present at ca 3200 m shifted between 1800 m glacials and ca 3500 m interglacials corresponding to a variation in temperature between ca 5 and 15 c at 2550 m altitude a provisional land sea correlation funza pollen odp site 677 18o is shown for the upper 1 2 ma stages 3 35 frequency analysis of several time series showed significant periods of the eccentricity 100 ka and precession 23 and 19 ka bands showing orbital forcing and a strong change in climatic variability around 800 ka at ca 2 7 ma a significant cooling of ca 5 c is documented reflecting the classical terrestrial pliocene pleistocene boundary which correlates to the reuverian praetiglian boundary of the nw european stratigraphical climatic subdivision

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

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

paleoelevation records from lipid biomarkers application to the tropical andes new results from two novel lipid biomarker based proxies help to constrain the late cenozoic surface elevation history of the eastern cordillera in the tropical northern andes of colombia previous well known studies have suggested rapid latest miocene pliocene 6 3 ma uplift on the basis of an abrupt shift in pollen species assemblages within sedimentary basin fill of the elevated bogota plateau from resampling of these original study localities we provide a revised chronology based on magnetic polarity stratigraphy and we evaluate paleotemperatures using the mbt cbt indices methylation of branched tetraethers cyclization of branched tetraethers and hydrogen isotopic composition d of leaf waxes as two independent proxies of past surface elevation reconstructed paleotemperatures from the mbt cbt proxy show a more gradual cooling trend from ca 7 6 ma to present consistent with less than 1000 m of elevation gain since latest miocene pliocene time and in agreement with geologic evidence for accelerated shortening and exhumation at this time the leaf wax isotopic data on the other hand lack a systematic trend potentially due to fractionation changes associated with uplift induced turnover in floral populations such changes could obscure isotopic variations in meteoric water suggesting that leaf wax isotopic compositions may not provide a direct proxy for elevation in this particular situation involving uplift in a heterogeneous tropical environment more promising in this case is the mbt cbt proxy which may offer a suitable alternative to carbonate based paleoelevation proxies in tropical regions where year round high precipitation rates prevent formation of soil carbonates

major altitudinal shifts in andean vegetation on the amazonian flank show temporary loss of biota in the holocene the lateglacial and holocene vegetation history from the amazonian flank of the south colombian andes has been studied in a 12 m long sediment core from lake la cocha 2780 m altitude 18 ams 14c

montane forest evolution during the last 650 000 yr in colombia a multivariate approach based on pollen record funza i the core interval of 0 140 m of the 357 m long pollen record from site funza i was analysed using multivariate analysis this long continental record is from the high plain of bogota located at 2550 m altitude in the eastern cordillera of colombia time control is based on a visual match with core funza ii from the same location which includes absolute fission track ages and a match with the 18o record of odp site 677 we reassessed the time frame of funza i compared with previous papers and conclude that the period of ca 650 25 kyr bp is represented in this study 73 fossil taxa present in 4 of the 409 pollen assemblages were analysed using the divisional classification program twinspan we recognised 10 cluster types that form three main cluster zones w warm cw cool and c cold pollen zones based on previous visual inspection were compared with new cluster types which were graphed as a bar code along the pollen diagram the use of two pollen sums standard pollen sum and a zonal pollen sum appeared useful for identifying local effects on the forest composition modern pollen rain data for the study area were summarised we used the concept of indicator taxa that behave as differential taxa to reconstruct climatic change during the period of 650 330 kyr bp alnus hedyosmum weinmannia melastomataceae myrica and podocarpus were the most important taxa of the andean forest the new time frame indicates that the first scattered presence of quercus started 478 kyr bp 102 m core depth the first continuous presence was from 423 kyr bp 92 m and the presence of zonal quercus forest occurred from 330 kyr bp 72 m during the period of 330 135 kyr bp zonal quercus and weinmannia forest developed replacing partly podocarpus dominated forest in the uppermost andean forest belt as a consequence during the last 135 kyr bp a high representation of quercus on the high plain of bogota no longer coincides with warm climatic conditions but now occurs during cool to cold conditions the pollen record was correlated with the 18o record temperature drop during marine oxygen isotope stage 4 and the very first part of marine oxygen oxygen stage 2 is calculated as a minimum of 6 6 to 7 8 c excluding a potential effect of low glacial pco2 the inferred maximum amplitude of the upper forest line during the last 650 kyr was from ca 1900 to 3400 m cf 3200 m at the present day corresponding to a range of ca 1 2 c to 7 8 c compared with today copyright 2000 john wiley sons ltd

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

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

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

forests of the tropical eastern andean flank during the middle pleistocene inter bedded volcanic and organic sediments from erazo ecuador indicate the presence of four different forest assemblages on the eastern andean flank during the middle pleistocene radiometric dates 40ar 39ar obtained from the volcanic ash indicate that deposition occurred between 620 000 and 192 000 years ago examination of the organic sediment composition and the fossil pollen wood and charcoal it contains provides insight into depositional environment vegetation assemblage and fire history the high organic content and abundance of macro fossils found throughout the sediment suggest that during the period of deposition the local environment was either a swamp or a shallow water body the correlation of fire activity peaks in charcoal abundance with volcanic ash deposits through most of the record suggests that volcanoes were the main source of ignition the low abundance of grass typically 10 throughout the sedimentary sequence along with the low abundance of other taxa indicative of open vegetation suggests the persistence of forest at erazo four types of forest assemblage were identified with the first taxa as the most dominant i alnus arecaceae ii miconia melastomataceae combretaceae moraceae urticaceae iii arecaceae alnus and iv podocarpus with oreopanax sp and melastomataceae combretaceae changes in the forest floristic composition indicate high vegetation turnover and reassortment of taxa between upper and lower montane forests during the middle pleistocene as well as the persistence of forest cover

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

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

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

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

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

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

slash burn and churn landscape history and crop cultivation in pre columbian amazonia abstract palaeoecologists argue that the vegetation of amazonia has changed markedly since the late pleistocene and show that forest fires have been increasingly more common since the beginning of the holocene because these occurrences coincide with the onset of human colonisation of the region students of amazonia discuss the extent to which it s holocene fire record reflects climatically related and or anthropogenic causes one factor that complicates inferences is the overall heterogeneity of the basin s vegetation during drier moments of the holocene and or within less humid sub regions of the basin it is difficult to conclude whether fire signals reflect more frequent natural fires or more intense burning by human communities another factor is that pre columbian livelihoods were not homogenously distributed throughout the region and changed over the course of the holocene this paper seeks to help establish an anthropogenic baseline against which holocene evidence for burning can be assessed to this end it reviews the role that slash and burn cultivation has had in discussions about pre columbian amazonia discusses how this account has been modified by recent research on pre columbian anthropogenic soils and queries the extent to which the regionally heterogeneous timing of crop domestication and cultivation may have influenced the fire history of pre columbian amazonia

transient quaternary erosion and tectonic inversion of the northern range trinidad abstract cosmogenic 10be measured in sediments from northern range catchments on the island of trinidad reveals low millennial scale rates of erosion average 40 mm ka that increase roughly eightfold from 11 to 92 mm ka eastward across the mountain range these results in conjunction with an analysis of mountain morphometrics are consistent with quaternary east side up tilting of the northern range which has occurred recently 100 ka the highest millennial scale erosion rates coincide spatially with quaternary east side surface uplift albeit not in magnitude high modern rainfall rates low topographic relief and convex longitudinal stream profiles indicating transient quaternary erosion we interpret that hillslope adjustment and erosion of the northern range is buffered from climatic and tectonic forcings by dense tropical vegetation cover thick saprolite and porous regolith compared with longer term thermochronology exhumation rates we document that an order of magnitude deceleration of and reversal in the locus of erosion and exhumation has occurred during the pliocene to the holocene we suggest that these combined data sets highlight distinct syn and post collisional phases of northern range development that are related to a major change in relative caribbean south american plate motion from oblique convergence to transform motion oblique collision during the mid miocene likely caused relatively higher rates of and asymmetric exhumation of the northern range post collisional mountain scale tilting is likely caused by a combination of crustal extension in the gulf of paria and by deep subsurface lithospheric detachment processes that drives dynamic topography

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

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

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

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

a 370 000 year record of vegetation and fire history around lake titicaca bolivia peru fossil pollen and charcoal analyses of sediments from lake titicaca peru bolivia provide a record of palaeoclimatic variation spanning four full glacial cycles pollen aquatic microfossils and charcoal as well as previously published data including diatom assemblages carbonate content and stable carbon isotopic ratios of organic carbon indicate that interglacials were warm and dry whereas the peaks of glacials were cold and wet each of the interglacials documented in the record are somewhat different with those of mis 5e and mis 9 inducing lower lake levels and a drier vegetation signature than those of mis 7 and 1 the presence of charcoal particles in sediments deposited during previous interglacials provides evidence of the long term role of fire in shaping andean ecosystems

paleoecological and archaeological implications of a late pleistocene early holocene record of vegetation and climate from the pacific coastal plain of panama abstract a phytolith record from monte oscuro a crater lake located 10 m above sea level on the pacific coastal plain of panama shows that during the late pleistocene the lake bed was dry and savanna like vegetation expanded at the expense of tropical deciduous forest the modern potential vegetation a significant reduction of precipitation below current levels was almost certainly required to effect the changes observed core sediment characteristics indicate that permanent inundation of the monte oscuro basin with water occurred at about 10 500 14c yr b p pollen and phytolith records show that deciduous tropical forest expanded into the lake s watershed during the early holocene significant burning of the vegetation and increases of weedy plants at ca 7500 to 7000 14c yr b p indicate disturbance which most likely resulted from early human occupation of the seasonal tropical forest near monte oscuro and the development of slash and burn methods of cultivation

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

influence of the amazon river on the nd isotope composition of deep water in the western equatorial atlantic during the oligocene miocene transition highlights records of seawater endend are highly unradiogenic across the omt at ceara rise strong influence of amazon particulate nd on seawater endend at ceara rise point sourced riverine nd influences seawater far from continental shelf regional sedimentary nd flux can be confused with water mass mixing signals abstract dissolved and particulate neodymium nd are mainly supplied to the oceans via rivers dust and release from marine sediments along continental margins this process together with the short oceanic residence time of nd gives rise to pronounced spatial gradients in oceanic 143nd 144nd ratios endend however we do not yet have a good understanding of the extent to which the influence of riverine point source nd supply can be distinguished from changes in mixing between different water masses in the marine geological record this gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large scale ocean water mass mixing but also with important changes in continental hydroclimate and weathering here we present endend data for fossilised fish teeth planktonic foraminifera and the fe mn oxyhydroxide and detrital fractions of sediments recovered from ocean drilling project odp site 926 on ceara rise situated approximately 800 km from the mouth of the river amazon our records span the mi 1 glaciation event during the oligocene miocene transition omt 23 ma we compare our endend records with data for ambient deep atlantic northern and southern component waters to assess the influence of particulate input from the amazon river on nd in ancient deep waters at this site endend values for all of our fish teeth foraminifera and fe mn oxyhydroxide samples are extremely unradiogenic end 15end 15 much lower than the endend for deep waters of modern or oligocene miocene age from the north atlantic end 10end 10 and south atlantic end 8end 8 this finding suggests that partial dissolution of detrital particulate material from the amazon end 18end 18 strongly influences the endend values of deep waters at ceara rise across the omt we conclude that terrestrially derived inputs of nd can affect endend values of deep water many hundreds of kilometres from source our results both underscore the need for care in reconstructing changes in large scale oceanic water mass mixing using sites proximal to major rivers and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering

late quaternary paleosol sediment sequences and landscape evolution along the andean piedmont bolivian chaco abstract as part of the bolivian chaco the andean piedmont is situated in the transitional zone between the tropical humid climate regime to the north amazonia and the subtropical semiarid regime to the south gran chaco in this zone which is sensitive to climatic changes late quaternary landscape and climate history was reconstructed based on detailed sedimentological pedological and geochemical analyses of paleosol sediment sequences along the andean piedmont the determination of characteristic lithofacies allowed correlation between the investigated profiles and their interpretation in terms of paleoenvironmental conditions i e geomorphic stability soil formation or geomorphic activity fluvial or fluvio aeolian sedimentation erosion etc coarse fluvial deposits gravels sands form the base of the investigated profiles and have likely been deposited in a braided river environment with low vegetation cover and intensive discharge events before and during the last glacial maximum well developed paleosols formed in the upper part of these sediments during the lateglacial documenting geomorphic stability high vegetation cover and wet conditions overlying palustrine sediments dated to the early holocene indicate a dramatic change in seasonality i e increased winter precipitation and decreased summer precipitation subsequent erosion followed by accumulation of fluvial and fluvio aeolian sands points to much drier conditions during the mid holocene since 2 9 cal ka bp soil formation indicates a return to geomorphic stability and wetter conditions characteristic of the present day climate interrupted by short arid intervals

influence of the andes mountains on south american moisture transport convection and precipitation mountain ranges are known to have a first order control on mid latitude climate but previous studies have shown that the andes have little effect on the large scale circulation over south america we use a limited domain general circulation model regcm3 to evaluate the effect of the andes on regional scale atmospheric dynamics and precipitation we present experiments in which andean heights are specified at 250 m and 25 50 75 and 100 of their modern values our experiments indicate that the andes have a significant influence on moisture transport between the amazon basin and the central andes deep convective processes and precipitation over much of south america through mechanical forcing of the south american low level jet llj and topographic blocking of westerly flow from the pacific ocean when the andes are absent the llj is absent and moisture transport over the central andes is mainly northeastward as a result deep convection is suppressed and precipitation is low along the andes above 50 of the modern elevation a southward flowing llj develops along the eastern andean flanks and transports moisture from the tropics to the subtropics moisture drawn from the amazon basin provides the latent energy required to drive convection and precipitation along the andean front large northerly moisture flux and reduced low level convergence over the amazon basin leads to a reduction in precipitation over much of the basin our model results are largely consistent with proxy evidence of andean climate change and have implications for the timing and rate of andean surface uplift

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

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

late pleistocene temperature depression and vegetation change in ecuadorian amazonia abstract paleoecological pollen phytolith and wood analyses of sediments radiocarbon dated 33 000 to 26 000 yr b p from two sites in ecuadorian amazonia provide data that suggest a cooling of ca 7 5 c below present in equatorial lowlands from 33 000 to 30 000 yr b p a period of warning followed in which novel species assemblages a blend of montane and lowland floral components persisted for at least 4000 years these data of forest community change from sites lying within the postulated glacial rain forest napo refugium provide the strongest paleoecological refutation of the refugial hypothesis yet obtained the large temperature depression at ca 30 000 yr b p allows the possibility that if maximum cooling at the equator was synchronous with the last glacial maximum lgm of the northern hemisphere freezing temperatures would have been experienced in parts of lowland amazonia between 25 000 and 18 000 b p

late quaternary environmental and climatic changes in central brazil paleoenvironmental changes dating back to 30 000 yr b p documented in a pollen record from central brazil lat 19 s permit the reconstruction of climatic changes related to shifts of the antarctic polar fronts the paleoclimatic inferences were obtained by a study of modern vegetation and pollen distribution taking into account present day climatic parameters at 30 000 yr b p the climate must have been warmer and moister than today judging from the high amount of tree pollen taxa characteristic of floodplain forest from 17 000 to 14 000 yr b p the climate was drier although tree pollen percentages were relatively high after 12 000 yr b p araucaria forest elements increased suggesting a moister and cooler climate the araucaria forest disappeared during a short interval between 11 000 and 10 000 yr b p this could be related to the younger dryas event at the beginning of the holocene the climate became cool and moist again as indicated by the reexpansion of the araucaria forest the latter was progressively replaced by a mesophytic semideciduous forest indicating warmer and drier climate after 8500 yr b p at 5000 yr b p an arid interval was followed by the expansion of mesophytic semideciduous forest elements

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

late quaternary vegetation and climate change in the panama basin palynological evidence from marine cores odp 677b and tr 163 38 abstract the late quaternary paleoenvironmental history from pacific slopes of the western andes is reconstructed by pollen analysis of 32 samples from two marine sediment cores from the panama basin eastern equatorial pacific core odp 677b 83 44 2200 w 1 12 1440 n 3473 m water depth is 185 cm long and spans the last 39 410 years core tr 163 38 81 583 w 1 337 n 2200 m water depth is 103 cm long and covers the last 17 380 years six ecological groups were established mangrove brackish and fresh water swamps terra firma lowland forests broad range taxa andean forests and open vegetation a good correspondence was found between the changes of these ecological groups in the two cores the records evidence the continuous presence of all vegetation types during the last 39 410 years and specially the uninterrupted occurrence of tropical rain forest they record a development from 1 a cold and humid phase 39 410 28 120 yr cal bp with moderately high sea levels 2 the coldest and driest phase in the record 28 120 14 500 yr cal bp accompanied by the lowest sea levels 3 a transitional phase when sea level rose and humid conditions dominated 4 a stage 11 300 5600 yr cal bp of the highest sea levels and moisture conditions including a drier period 7000 yr bp to 5 a final period 5600 yr cal bp present when sea level reached its present height humidity persisted and indicators of disturbance expanded peaks in pollen and spore concentration associated with high river discharge periods indicate periods of higher precipitation around 33 500 28 000 and 12 000 9000 yr cal bp although main vegetation responses seem to reflect rainfall and moisture variations a good correspondence was found between 18o values and percentages of andean and lowland pollen suggesting that vegetation also responded to tempearture changes

modern pollen signatures of amazonian rivers and new insights for environmental reconstructions abstract paleoenvironmental reconstructions based on amazonian fluvial and marine pollen data are often hindered due to the scarcity of modern calibration data in amazonia rivers are the main pathways for pollen from upland and flooded forests to continental and marine sediment sinks linking pollen assemblages transported by rivers to watershed vegetation is crucial to interpret palynological archives in order to constrain the interpretation of palynological archives we analysed the pollen content of 36 riverbed sediment samples distributed from the mouth of the ica river ca 68ow to the amazon estuary ca 49ow with support of grain size and major element analyses of the inorganic sedimentary content the palynological analysis shows a strong imprint of the lowland floodplain vegetation as the main source of the riverborne pollen the upper reaches of the amazon river present abundant cecropia reflecting early successional stages induced by intense floodplain disturbance the middle and lower reaches of the amazon river exhibit a progressive increase in herbs mainly represented by poaceae and cyperaceae related to wider floodplains covered by seasonally inundated vegetation floodplain forests of whitewater rivers varzeas are palynologically characterized by iriartea mauritia ilex pseudobombax acalypha and luehea laetia with high frequency of cecropia or herbs and montane taxa at the black and clearwater rivers the igapo floodplain forests provide high percentages of symmeria myrtaceae and alchornea and can be distinguished by sapium sebastiana piranhea amanoa pouteria ferdinandusa genipa attalea astrocaryum and eschweilera which suggest conditions of higher stability and mature forests contribution from the non flooded uplands andean mountains and savannas cerrado occur but their signals are blurred by the high pollen production of the lowland floodplains our data provide a calibration set for environmental reconstructions of river influenced deposits across the amazon including offshore areas of the most iconic riverine dominated landscape in the world

frequency spectra and paleoclimatic variability of the high resolution 30 1450 ka funza i pollen record eastern cordillera colombia the evolution of orbital periodicities during the quaternary has been studied in the 30 1450 ka funza i pollen record from cores through the sedimentary basin of bogota eastern cordillera colombia the 1178 pollen samples analysed between 2 90 and 340 00 m generate 5 time series with an average time resolution of ca 1200 years our geochronological framework is based on fission track ages on zircon in combination with correlation of the pollen record with the oxygen isotope record of odp site 677 from the eastern pacific the 5 data sets were analysed in the frequency domain with maximum entropy spectrum analysis mesa and thomson multi taper spectrum analysis tmt using a moving 400 ka window with steps of 20 ka frequencies closely related to the milankovitch theory were detected in all 5 data sets the 100 ka period in the arboreal pollen record is restricted to the last ca 800 ka of the quaternary at a confidence level largely higher than 90 periodicities close to 23 ka occur throughout the record at a high significance level above 90 and are strongest in the subparamo data a statistically significant period centred at ca 40 ka also occurs in the funza i record four frequency bands centred at ca 30 ca 15 ca 13 and ca 10 ka were also detected the 5 principle data sets from the funza i pollen record reveal an evolution of orbital frequencies in the middle and late quaternary which are in general agreement with other marine e g imbrie et al 1984 and continental e g kukla et al 1990 records

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 late pleistocene early holocene archaeological record in brazil a geo referenced database abstract between 13 000 and 8000 14c bp eastern south america was settled by a stable and diversified population of hunter gatherers archaeological excavation in the past twenty years has yielded increasingly consistent evidence of occupation in different regions of brazil since the end of the pleistocene with dates at least contemporary to the clovis horizon in north america this is addressed by documenting and analysing the quantity quality and distribution of archaeological 14c dates from brazil during this period a total of 277 dates from 90 sites are tabulated mapped and included in the analysis during the late pleistocene there was a pioneer phase of human colonization with dispersal inland through the major river systems subsequently the early holocene saw the first phase of established settlement of brazil s interior there seems to be an archaeological threshold reached at ca 10 500 years 14c bp numbers of sites increase there is evidence of settlement of all major biomes and there is clear evidence of inter regional cultural diversity

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

late holocene environmental history of southern choco region pacific colombia sediment diatom and pollen analysis of core el caimito abstract we present a multi proxy study of pollen diatoms sediment characteristics and major elements of a 610 cm sediment core from lake el caimito located in the humid rain forest of southern choco pacific colombia we propose an integrated reconstruction of the local basin development and of the regional vegetation which is possibly related to the tectonic activity of an unstable coastal area time control is based on 7 ams 14 c dates that show that the record represents the last 3850 cal yr bp from 3850 to 2700 cal yr bp sandy deposits low carbon content absence of diatoms and low diversity of the pollen spectra indicate that the site was under the influence of the fluvial system erosive event s removed part of the sediment record and we observed a hiatus representing 700 years after that the basin became more isolated from the river drainage system from 2010 to 1430 cal yr bp mainly clay was deposited and repeatedly interrupted by river pulses that left sandy and silty horizons in the record benthic and littoral benthic diatom species indicate a shallow water body and a stable water chemistry mangrove forest was close to the lake apparently growing along the close by inlets regionally the main vegetation elements were palms and taxa in the families of moraceae urticaceae melastomataceae fabaceae and from a number of other families and genera characteristic of tropical lowland rain forest from 1430 to 810 cal yr bp the river impact gradually diminished each fluvial event that affected the local forest is shown in the pollen record by an expansion of cecropia dominated pioneer forest decreasing intensity of forest disturbance coincides with an increase in the diversity of fossil pollen taxa possibly reflecting an increasing plant diversity of the forest mangrove pollen declined indicating that the coastline moved seaward and suggesting tectonic uplift of the coastal area between 810 and 580 cal yr bp mangrove forest was closer to the lake again reflecting an inland migration of the coastline suggesting tectonic subsidence from 580 to 300 cal yr bp the last fluvial events were recorded diatom associations indicate oligotrophic and acidic water the mangrove belt moved seaward again suggesting tectonic uplift palms and cecropia became more abundant suggesting increased human impact in the near shore lowland forest during the last 300 years stable lacustrine conditions and lowland rain forest with the highest floral diversity is registered

holocene paleolimnological reconstruction of a high altitude colombian tropical lake abstract aquatic environments in the high andean may be quite vulnerable to climatic conditions and hydrological processes precipitation evaporation that have the potential to alter water levels and chemistry this study reconstructs the paleohydrological history of lake tota colombia to provide a record of environmental changes in the eastern flank of the colombian andes during the late holocene a 54 cm core was collected at the margin of the lake at a depth of 1 5 m and the paleohydrological reconstructions were based on changes in diatom assemblages granulometry and bulk organic geochemical parameters the core was subdivided into 5 biozones by assessing the significance of the variance among the coniss clusters biozone t5 base 4000 calibrated years bp was dominated by the planktonic diatom discostella stelligera and was deposited in a period of high water level sedimentation during this period was rich in carbon due to phytoplanktonic production the granulometry data suggests that the lake was under high to moderate hydrological dynamic conditions indicating active deposition at the river delta front biozone t4 beginning at 3600 cal yrs bp was dominated by three species d stelligera cocconeis placentula var lineata and fragilaria sensu lato and this association could indicate that the lake was under the influence of large fluctuations in water level this interpretation is supported by granulometry and geochemical data particularly by the low levels of total organic carbon toc in biozone t3 corresponding to 3000 cal yrs bp we found a lowering of the water level suggested by low levels of toc poor granulometric sorting and higher carbon nitrogen c n ratios suggest a macrophyte influence on the sedimentation avoiding direct deltaic influence in this biozone the benthic diatom f sensu lato group was dominant in biozone t2 starting 2400 cal yrs bp the proxies indicate that the lake experienced increasing water levels and that stronger circulation patterns occurred in the water column than in the preceding periods as inferred by the presence of aulacoseira granulata a ticoplanktonic species the sediment in this biozone is well sorted and finer indicating a greater depth biozone t1 at the top of the core and since 2000 cal yrs bp to the present evidence for a recent reduction in water level of lake tota can be deduced by the presence of achnanthidium minutissimum var minutissimum

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

late quaternary vegetation and coastal environmental changes at ilha do cardoso mangrove southeastern brazil abstract a 190 cm mangrove sediment core from the ilha do cardoso state park state of sao paulo southeastern brazil was analyzed for pollen diatoms as well as carbon and nitrogen isotopes the goal was to determine the dynamics of the coastal terrestrial aquatic ecosystems vegetation history and climate change in this region of the brazilian atlantic rainforest during the late pleistocene and holocene the values for total organic carbon toc from 3 up to 40 c n ratios from 10 up to 130 and 15n 0 to 8 are associated with well preserved aquatic and terrestrial organic matter and possibly influenced by nitrogen cycling e g denitrification that caused 15n enrichment between 40 000 cal yr b p and 23 000 cal yr b p depleted 13c values 28 0 are also observed and indicate the predominance of c3 plants during this time interval the pollen analysis reveals the presence of the genera ilex alchornea weinmannia myrsine symplocos drimys and podocarpus on a site currently occupied by mangrove vegetation these data suggest that in the past prevailed a colder and more humid climate than today with a low relative sea level from 23 000 cal yr b p to 2200 cal yr b p a sedimentary hiatus likely occurred related to an erosive event associated to the post glacial sea level rise since at least 2200 cal yr b p sediments are marked by relatively low c n ratios from 2 to 27 exhibit more enriched 13c from 26 0 to 24 0 and 15n up to 7 values and are characterized by the presence of marine diatoms this indicates the return of the marine coastal line to its current position and consequently the development of mangrove

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

an optical luminescence chronology for late pleistocene aeolian activity in the colombian and venezuelan llanos the lowland savannas llanos of colombia and venezuela are covered by extensive aeolian landforms for which little chronological information exists we present the first optically stimulated luminescence osl age constraints for dunes in the llanos orientales of lowland colombia and new ages for dunes in the venezuelan llanos the sampled dunes are fully vegetated and show evidence of post depositional erosion ages range from 4 5 0 4 to 66 4 ka with the majority dating to 27 10 ka marine isotope stage 2 some dunes accumulated quickly during the last glacial maximum although most were active 16 10 ka accretion largely ceased after 10 ka all dunes are elongated downwind from rivers parallel with dry season winds and are interpreted as source bordering features as they are presently isolated from fluvial sediments by gallery forest it is proposed that activity was associated with a more prolonged dry season which restricted gallery forest leading to greater sediment availability on river shorelines such variability in dry season duration was potentially mediated by the mean latitude of the itcz the cessation of most dune accretion after ca 10 ka suggests reduced seasonality and a more northerly itcz position consistent with evidence from the cariaco basin

vegetation invasions into absolute desert a 45 th000 yr rodent midden record from the calama salar de atacama basins northern chile lat 22 24 s plant macrofossils percentage abundance of grass taxa fecal pellet d13 c and plant cuticle contents from 49 fossil rodent middens dated by 14 c record changes in local vegetation and precipitation since 45 ka calibrated or measured to thousands of calendar years before present in the central atacama desert lat 228 248s of northern chile the midden sites are along the hyperarid upper margin 2400 3100 m of the absolute desert in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs conditions between 40 and 22 ka may have been at least intermittently dry and possibly cooler as implied by four middens with low species richness we infer a large increase in summer rainfall between 16 2 and 10 5 ka on the basis of the lowering of steppe grasses by as much as 1000 m prominence of c4 grasses and summer annuals high species richness and displacement of northern species at least 50 km south of their modern ranges the precipitation increase was greatest for a cluster of middens between 11 8 and 10 5 ka abrupt drying evident in a dramatic decrease in grass abundance occurred after 10 5 ka at all four midden localities increased percentages of