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

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

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

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

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

a 50 000 yr pollen record from chile of south american millennial scale climate instability during the last glaciation abstract high resolution 100 yr sampling interval chilean pollen data between 10 000 and 60 000 cal yr b p exhibit systematic fluctuations in subantarctic parkland development these variations are dominated by a 30 000 40 000 yr cycle similar to that in the northern hemisphere gisp2 18 o data and other climatic records both chilean and gisp2 data show oscillations in the 5000 12 000 and 1000 3000 yr period bands the coherence is however generally low and distribution of power spectra differs with the dynamics of the pollen assemblage dominated by a combination of 12 000 and 5000 yr cycles we suggest a preferential nonlinear response of chilean vegetation to climatic forcing and interhemispheric differences as possible mechanisms underlying the resemblances and dissimilarities between both records

landscape evolution during the late quaternary at the doce river mouth espirito santo state southeastern brazil article the sedimentary deposits of the deltaicplain of the doce and barra seca rivers southeastern brazil were studied by faciesanalysis pollenrecords 13 c c n analysis and ams 14 c dating today thisdeltaicplain isdominated by beach ridges and sandy terraces occupied by arboreal and herbaceous vegetation between 47 500 and 29 400 cal yr b p a deltaic system was developed in response mainly to eustatic sea level fall although the studied stratigraphic succession is compatible with the trend of global sea level fall the earlier sea level sug gested by the topographic position of these deltaic deposits was above the one expected during the mis3 stage a tectonic uplift likely occurred during the late quaternary and raised these deposits the post glacial sea level rise caused a marine incursion with invasion of embayments and broad valleys and it favored the evolution of an estuary with wide tidal mud flats occupied by mangroves between 7400 and 5100 cal yr b p the high river sand supply and or the relative sea level fall in the late holocene led to seaward and downward translation of the shoreline during normal forced regression producing progradational deposits with shrinkage of mangrove stands and expansion of marshes colonized by herbaceous vegetation therefore the stratigraphic architecture and evolution of the doce river deltaic plain suggest that fluvial sediment supply and relative sea level fluctuations related to quaternary global climatic changes and local tectonism exerted major controls on sedimentation through the variation of accommodation space and base level changes

orbital and internal forcing of climate on the yucatan peninsula for the past ca 36 ka abstract a 19 6 m sedimentary sequence from lake quexil guatemala is presented as a proxy record of climatic change on the low lying yucatan peninsula over the last ca 36 ka long term climatic fluctuations are attributed to orbital forcing of insolation while abrupt changes during the late glacial period are the result of internal forcing mechanisms that remain imperfectly modelled gcms simulating meltwater cooling of the gulf of mexico may predict some but not all climatic responses in the circum caribbean lake quexil did not experience greater aridity than predicted by orbital forcing during late glacial zones q lg 1 and 2 which were drier only in comparison with holocene conditions q lg 1 appears to have been colder than expected based on insolation alone and is contemporary with the younger dryas event between meltwater pulses 1a and 1b thus other feedbacks also may have cooled surface temperatures in the gulf of mexico such as unique late glacial circulation patterns increased albedo for the florida and yucatan peninsulas or surface temperatures on the north american continent additional terrestrial sequences are needed to improve the chronology and confirm the distribution of sub regional climatic variability in the circum caribbean

source and distribution of pollen and spores in surface sediments of a plateau lake in southeastern amazonia abstract in the past two decades several paleobotanical studies on lakes have indicated a drier climatic condition in amazonia during the last glacial maximum lgm and early mid holocene which likely resulted in the contraction of the tropical forest and expansion of savanna however these investigations in the eastern amazon are scarce and the reliability and precision of the inference remains largely limited in this study palynological analyses were carried out in surficial sediments of violao lake southeastern amazonia to assess the distribution and sources of pollen and spores from montane savanna and forest based on surrounding vegetation types catchment basin and wind patterns and also to identify modern pollen rain characteristics of the ecosystems developed under edaphic conditions considering the spatial distribution of pollen and spores in the lake basin the montane savanna pollen are predominant in the eastern and western borders and subordinate in the central portion of the lake whereas those of forest formations are mainly present in its western margin pteridophytes and algae spores show a distinct surface distribution compared to montane savanna pollen while the algae spores have a good correlation with forest formation pollen the correlation of these data along with vegetation types of the catchment basin indicates that the absence of pollen grains from species that present large occurrence along the catchment basin may be partially related to phenological characteristics that avoided pollen losses however in general montane savanna pollen in the bottom sediments tends to represent a signal of the regional vegetation developed over ferruginous duricrusts in the serra do carajas while forest formation pollen may be used to identify changes in forest structure of the catchment basin of violao lake as montane savanna is developed under edaphic conditions in hydric stress substrate a systematic description of pollen grains clearly demonstrates that only changes in forest vegetation can be used as a reliable indicator of paleoclimate changes

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

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

late quaternary vegetational and climatic changes in cerrado and palm swamp from central brazil abstract geochemical and palynological analyses of core cr1 from crominia goias provided information on the paleoenvironmental history of central brazil the records show that prior to 32 400 yr b p the vegetation of the crominia region was similar to the present one characterized locally by a complex of cerrado gallery forest and palm swamp indicating a semi humid tropical climate a palm swamp occupied the coring site from 32 400 to about 20 000 yr b p a treeless grassland replaced the arboreal cerrado and the gallery forest suggesting that humidity increased but temperature probably decreased the site was a shallow lake between ca 27 000 and 20 000 yr b p humidity started to decrease at ca 18 500 yr b p and the period from ca 18 500 to ca 11 500 yr b p was very dry a sparse vegetation was growing in the region during that time the dry climate continued until 6500 yr b p and the core site probably dried out several times at ca 5000 yr b p humidity increased again the palm swamp vegetation returned to the site and cerrado vegetation and gallery forest started to grow in the region the abundant charcoal particles prior to 20 000 yr b p and from 10 500 to ca 3500 yr b p document a long history of fires in the region results are compared with those from other sites in central brazil and with the climate sequence of the last glacial maximum and postglacial time in the tropical andes

near surface circulation in the northern north atlantic as inferred from lagrangian drifters variability from the mesoscale to interannual 1 the near surface circulation of the nordic seas is basically cyclonic and consists of jets and recirculation cells which are tightly linked to the bottom topography variable forcing by the large scale rotation of the wind leads to a modulation in the strength of the gyres and their interconnecting jets this is seen in drifter and altimeter data currents are stronger during winter and during phases of high north atlantic oscillation index the exchanges between the north atlantic and the nordic seas do not seem to be directly affected by this variable forcing the narrow boundary currents and the intergyre jets are subject to instability causing mesoscale current fluctuations which contribute to the stirring and mixing of polar and atlantic water masses

ice core palaeoclimate records in tropical south america since the last glacial maximum ice core records spanning the last 25 000 yr from the tropical andes of south america are reviewed these records from quelccaya huascaran and sajama present a high temporal resolution picture of both the late glacial stage lgs and the holocene climatic and environmental conditions in the south american andes late glacial stage conditions at high elevations appear to have been cooler than today although the magnitude of the inferred cooling differs with the particular proxy used e g snowline depression pollen ice cores insoluble dust and anion concentrations in the ice cores reveal that lgs hydrological conditions in the tropics 9 s were much drier than today whereas in the subtropics 18 s lgs conditions were much wetter this probably reflects the migration of the tropical hadley cell in response to a different meridional temperature gradient low nitrate concentrations in the lgs ice from both huascaran and sajama suggest that the amazon basin forest cover may have been much less extensive discussed is the conundrum surrounding the use of d 18 o as a palaeothermometer in the tropics where temperatures exhibit little seasonal variation yet the ice core records suggest that d 18 o records temperature variations on decadal to millennial time scales finally evidence is presented for a strong twentieth century warming copyright 2000 john wiley sons ltd

late quaternary palaeoclimate in the savannas of south america the paper reviews late quaternary palynological evidence from eight sites in the savannas of northern south america the sites reviewed are lake valencia 10 n carajas 6 s aguas emendadas 15 s crominia 17 s salitre 19 s lagoa de serra negra 18 s lagoa santa 20 s and lagoa dos olhos 20 s four of the sites show evidence for a climate more humid than at present from ca 36 ka bp to ca 22 18 ka bp maximum dryness was reached at 14 ka to 10 5 ka bp at lake valencia there was an increase in moisture from 8 8 ka bp at lake valencia and from 7 ka bp in central brazil the presence of charcoal indicates human impact from ca 8 6 ka bp 1997 john wiley sons ltd

quantitative estimates of glacial and holocene temperature and precipitation change in lowland amazonian bolivia quantitative estimates of temperature and precipitation change during the late pleistocene and holocene have been difficult to obtain for much of the lowland neotropics using two published lacustrine pollen records and a climate vegetation model based on the modern abundance distributions of 154 neotropical plant families we demonstrate how family level counts of fossil pollen can be used to quantitatively reconstruct tropical paleoclimate and provide needed information on historic patterns of climatic change with this family level analysis we show that one area of the lowland tropics northeastern bolivia experienced cooling 1 3 c and drying 400 mm yr relative to present during the late pleistocene 50 000 12 000 calendar years before present cal yr b p immediately prior to the last glacial maximum lgm ca 21 000 cal yr b p we observe a distinct transition from cooler temperatures and variable precipitation to a period of warmer temperatures and relative dryness that extends to the middle holocene 5000 3000 cal yr b p this prolonged reduction in precipitation occurs against the backdrop of increasing atmospheric co2 concentrations indicating that the presence of mixed savanna and dry forest communities in northeastern bolivia durng the lgm was not solely the result of low co2 levels as suggested previously but also lower precipitation the results of our analysis demonstrate the potential for using the distribution and abundance structure of modern neotropical plant families to infer paleoclimate from the fossil pollen record

early human occupation and land use changes near the boundary of the orinoco and the amazon basins se venezuela palynological evidence from el pauji record this paper shows a holocene paleoecological reconstruction based on a peat bog sequence el pauji 4 28 n 61 35 w 865 m elevation located in the transition zone between the gran sabana se venezuela savannas and the amazon rainforests paleoecological trends are based on the analysis of pollen and pteridophyte spores algal and animal remains fungal spores and charcoal particles the whole record embraces the last ca 8000 cal yr bp and was subdivided into five pollen zones representing the following vegetation succession savanna rainforest mosaic 8250 7715 yr bp dense rainforests 7715 5040 yr bp savanna rainforest mosaic 5040 2690 yr bp secondary dry forests 2690 1440 yr bp and peat bog in an open savanna landscape 1440 yr bp present these vegetation changes have been attributed to the action of climate and or land use changes as well as the corresponding synergies between them fire has been determinant in the landscape evolution based on the reconstructed fire and vegetation shifts a changing land use pattern could have been recognized between the early and the mid holocene ca 8 3 5 0 kyr bp land use practices seem to have been more linked to shifting agriculture in a rainforest landscape as is usual in amazon cultures with medium fire incidence affecting only local forest spots or surrounding savannas more extensive forest burning was recorded between ca 5 0 and 2 7 kyr bp followed by land abandonment and the dominance of drier climates between 2 7 and 1 4 yr bp the modern indigenous culture which prefers open environments and makes extensive use of fire thus preventing forest re expansion seem to have established during the last 1500 yr therefore a significant cultural replacement has been proposed for the region leading to the present day situation changing human activities have been instrumental for ecological evolution in this savanna rainforest transitional region as well as for the shaping of modern landscapes

holocene fires in the northern amazon basin abstract the ubiquitous occurrence of charcoal in the forest soils of the upper rio negro region of colombia and venezuela indicates the presence of frequent and widespread fires in the amazon basin possibly associated with extremely dry periods or human disturbances charcoal ranged from 3 12 to 24 76 mg cm 3 in the upper 50 cm of soil and was more abundant in oxisols and ultisols than in other soil types charcoal dates range from 6260 yr b p to the present several dates coincide with dry phases recorded during the holocene ceramic shards were found at several sites and thermoluminescence analysis indicates that their ages range from 3750 to 460 yr b p the age of charcoal and shards confirms that this region has been subjected to fire and human disturbances during the past 6000 yr

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

use of multi proxy approaches to determine the origin and depositional processes in modern lacustrine sediments carajas plateau southeastern amazon brazil abstract geochemical and isotopic compositions of surficial sediments from a plateau lake in carajas southeastern amazon region were investigated to understand the spatial distribution of major and trace elements 13 c 15 n and c n ratio depositional processes and the origin of inorganic and organic fractions the 13 c 15 n and c n ratio indicate mainly an autochthonous source with siliceous sponge spicules and algae being the major source of organic matter in the center of the lake sector 3 while an allochthonous source mainly derived from c3 vascular forest plants dominates in the shallowest portion of the lake sector 1 consequently there was an apparent dilution of c4 plants montane savanna in the sediment water interface among major elements fe 2 o 3 is highly enriched in sector 3 which is controlled by the erosion of catchment laterites and underwater topography while al 2 o 3 and p 2 o 5 enrichment near the northern border of the lake is controlled by the weathering of mafic rocks similar spatial distribution of sio 2 with total organic carbon toc and isotopic evidence indicate that si distribution is partially controlled by organic components such as siliceous sponge spicules and algae the occurrence of most of the trace and rare earth elements ree is independent of fe 2 o 3 and toc but controlled by detrital aluminum silicates and heavy minerals indicating a lack of post depositional diagenetic control on their distribution the distribution of as and mo are possibly controlled by organic matter mineralization during early diagenesis and subsequently precipitation of fe phases the values of the chemical index of alteration cia in the sediments are very high 94 99 and similar to those of the source rocks suggesting that sediment composition is mainly controlled by mechanical weathering rather than chemical weathering geochemical indices al k ti k al ti la th ti zr zr hf hf nb la al co th ba sr and th sc together with the a cn k plot suggest that the provenance of the inorganic sediments remained relatively uniform or constant during the depositional period and that they were mainly derived from laterite crusts and subordinately from mafic soils this inference was further substantiated by the chondrite normalized ree patterns and discrimination plots geochemical indices such as u th authigenic u mo al and v cr indicate that the sediments were deposited under an oxic environment

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

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

high resolution climate signals in the bolling allerod interstadial greenland interstadial 1 as reflected in european tree ring chronologies compared to marine varves and ice core records abstract lateglacial and holocene tree ring chronologies are unique archives which provide various information on past environments on a true annual time scale changes in ring width can be related to past climate anomalies and dendrodated wood provides an ideal source for radiocarbon calibration we present a 1051 year tree ring chronology from the late glacial built from subfossil scots pines pinus sylvestris that grew in different regions of central and southern europe through a series of high precision radiocarbon measurements we obtained a floating radiocarbon chronology which allowed accurate wiggle matching to the intcal98 calibration curve the trees show a coherent pattern in ring width variations throughout central europe and extending into the mediterranean which indicates a strong external climatic factor most probably temperature during the growing season we identified major growth events which appear synchronous with events seen in isotopic and tracer signals in the greenland ice cores and with changes in the strength of upwelling in the cariaco basin

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

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

a 45 kyr palaeoclimate record from the lowland interior of tropical south america we present a well dated high resolution 45 kyr lake sediment record reflecting regional temperature and precipitation change in the continental interior of the southern hemisphere sh tropics of south america the study site is laguna la gaiba llg a large lake 95 km2 hydrologically linked to the pantanal an immense seasonally flooded basin and the world s largest tropical wetland 135 000 km2 lake level changes at llg are therefore reflective of regional precipitation we infer past fluctuations in precipitation at this site through changes in i pollen inferred extent of flood tolerant forest ii relative abundance of terra firme humid tropical forest versus seasonally dry tropical forest pollen types and iii proportions of deep versus shallow water diatoms a probabilistic model based on plant family and genus climatic optima was used to generate quantitative estimates of past temperature from the fossil pollen data our temperature reconstruction demonstrates rising temperature by 4 c at 19 5 kyr bp synchronous with the onset of deglacial warming in the central andes strengthening the evidence that climatic warming in the sh tropics preceded deglacial warming in the northern hemisphere nh by at least 5 kyr we provide unequivocal evidence that the climate at llg was markedly drier during the last glacial period 45 0 12 2 kyr bp than during the holocene contrasting with sh tropical andean and atlantic records that demonstrate a strengthening of the south american summer monsoon during the global last glacial maximum 21 kyr bp in tune with the 20 kyr precession orbital cycle holocene climate conditions occurred as early as 12 8 12 2 kyr bp when increased precipitation in the pantanal catchment caused heightened flooding and rising lake levels in llg in contrast to this strong geographic variation in lgm precipitation across the continent expansion of tropical dry forest between 10 and 3 kyr bp at llg strengthens the body of evidence for widespread early mid holocene drought across tropical south america

late quaternary vegetation climate and fire dynamics and evidence of early to mid holocene polylepis forests in the jimbura region of the southernmost ecuadorian andes abstract the vegetation climate and fire history of high mountain ecosystems in the southern ecuadorian andes are documented in a continuous pollen and charcoal record from lagunas natosas forest bog at 3495 m a s l spanning the late pleistocene and holocene periods since 15 930 cal yr bp the chronology of this record is based on four radiocarbon dates paramo vegetation with abundant poaceae and plantago rigida characterized the late glacial lg period implying cold and wet conditions slight expansions of mountain forests suggest warming during the lg the upper forest line was lower than today and fire events were rare during the pleistocene during the early and mid holocene until 4300 cal yr bp the area of paramo was markedly smaller and the subparamo with woody vegetation melastomataceae was larger the upper mountain forest dominated and the upper forest line shifted to higher elevations the pollen record provides the first evidence of the development of polylepis forests in the southern part of the so called andean depression reflecting warmer and drier climatic conditions during early and mid holocene the late holocene after about 4300 cal yr bp is characterized by larger areas of paramo and subparamo promoted by moister conditions these moister conditions and frequent fires may have been the cause of the very rare occurrence or even extinction of polylepis populations in the jimbura region since ca 1000 cal yr bp human occupation has altered the landscape by frequent burning wood cutting grazing and cultivation

changes in fire regimes since the last glacial maximum an assessment based on a global synthesis and analysis of charcoal data fire activity has varied globally and continuously since the last glacial maximum lgm in response to long term changes in global climate and shorter term regional changes in climate vegetation and human land use we have synthesized sedimentary charcoal records of biomass burning since the lgm and present global maps showing changes in fire activity for time slices during the past 21 000 years as differences in charcoal accumulation values compared to pre industrial there is strong broad scale coherence in fire activity after the lgm but spatial heterogeneity in the signals increases thereafter in north america europe and southern south america charcoal records indicate less than present fire activity during the deglacial period from 21 000 to 11 000 cal yr bp in contrast the tropical latitudes of south america and africa show greater than present fire activity from 19 000 to 17 000 cal yr bp and most sites from indochina and australia show greater than present fire activity from 16 000 to 13 000 cal yr bp many sites indicate greater than present or near present activity during the holocene with the exception of eastern north america and eastern asia from 8 000 to 3 000 cal yr bp indonesia and australia from 11 000 to 4 000 cal yr bp and southern south america from 6 000 to 3 000 cal yr bp where fire activity was less than present regional coherence in the patterns of change in fire activity was evident throughout the post glacial period these complex patterns can largely be explained in terms of large scale climate controls modulated by local changes in vegetation and fuel load

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

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

a long history of cloud and forest migration from lake consuelo peru abstract the complete paleoecological history from lake consuelo forest yields a record of ground level cloud formation and changes in its lower altitudinal limit over the last 46 300 cal yr bp the timing of early lake level fluctuations prior to 37 000 cal yr bp appears sensitive to north atlantic temperature oscillations corresponding to dansgaard oeschger interstadials 11 10 and 8 recorded in gisp2 after the lgm the first hint of warming is recorded in lake consuelo at 22 000 cal yr bp and agrees with other estimates for the region the mid holocene 7400 5000 cal yr bp was the period of highest rates of change and most significant reorganizations in the consuelo forest these community changes resulted from a regionally widespread dry period results from lake consuelo indicate that moisture availability mediated through cloud cover played the most significant role in ecological change in this system rates of past climate fluctuations never exceeded the forest capacity to accommodate change unfortunately this might not be the case under predicted scenarios for the end of the current century

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

vegetation changes in the neotropical gran sabana venezuela around the younger dryas chron the occurrence of the younger dryas cold reversal in northern south america midlands and lowlands remains controversial we present a palaeoecological analysis of a late glacial lacustrine section from a midland lake lake chonita 4 6501 8n 61 0157 8w 884 m elevation located in the venezuelan gran sabana based on physical and biological proxies the sediments were mostly barren from 15 3 to 12 7 k cal a bp probably due to poor preservation a ligneous community with no clear modern analogues was dominant from 12 7 to 11 7 k cal a bp younger dryas chronozone at present similar shrublands are situated around 200 m elevation above the lake suggesting a cooling driven downward shift in vegetation during that period the interval from 11 7 to 10 6 k cal a bp is marked by a dramatic replacement of the shrubland by savannas and a conspicuous increase in fire incidence the intensification of local and regional fires at this interval could have played a role in the vegetation shift a change to wetter and probably warmer conditions is deduced after 11 7 k cal a bp coinciding with the early holocene warming these results support the hypothesis of a mixed origin climate and fire of the gran sabana savannas and highlight the climatic instability of the neotropics during the late glacial copyright 2011 john wiley sons ltd

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

geomorphology as science the role of theory abstract because geomorphology is a science it is permeated by theory overt recognition of this actuality is frequently resisted by geomorphologists earth history does not represent an alternative to earth science it is an essential component of earth science in its broadest sense science seeks to discover new knowledge through a two stage activity involving the creation and justification of ideas theory deduction is generally regarded as the only logically consistent method of justifying ideas the creation of ideas is a much more controversial topic some methodologists deem it beyond logic certainly deductive arguments which are nothing more than formal logical expressions of theory play no role in the conception of new ideas many earth scientists generate possible explanations of observed phenomena based on abductive reasoning others advocate reliance on purported forms of pure induction such as serendipity and intuition in which observations assume primacy over theory besides lacking consistency and educability the latter posture is flawed because it mistakenly implies that becoming well versed in theory is irrelevant to or impedes scientific discovery irrational or subjective factors play a role in the creation of ideas but it is erroneous to claim that these factors are divorced from theory science is first and foremost a cognitive activity thus the primacy of observations in science is a myth all observations are theory laden in the sense that the act of observation inherently involves interpretation and classification both of which can only occur within the context of theoretical preconceptions even discoveries based on unexpected observations require the fortunate investigator to recognize the theoretical importance of what is seen or measured the most useful view of geomorphology as a science is one in which theory is seen as central but fragile and in which theory and observation are viewed symbiotically with theory providing the generative force and observation providing a vital policing role much of the current debate in geomorphology centers around differences in characteristics of theory type of scientific arguments and metaphysical perspectives among investigators working at different temporal scales full recognition and understanding of these differences are essential for developing a unified approach to the science of geomorphology

a 43 ka record of paleoenvironmental change in the central american lowlands inferred from stable isotopes of lacustrine ostracods we present a continuous ostracod isotope 18o and 13c record from lake peten itza peten guatemala in the northern lowland neotropics that spans the last 43 cal ka bp variations in oxygen and carbon isotopes closely follow lithologic variations which consist of alternating gypsum and clay deposits that were deposited under relatively dry and wet climate respectively during the last glacial period the greatest 18o and 13c values coincide with gypsum deposited during lake lowstands under arid climate conditions that were correlated previously with north atlantic heinrich events in contrast interstadials and the entirety of the last glacial maximum 24 19 cal ka bp are marked by clay deposition and lower 18o and 13c values reflecting higher lake levels and relatively moister climate isotope results and pollen data along with independently inferred past water levels show the early deglacial period 19 15 cal ka bp was the time of greatest aridity and lowest lake stage of the past 43 ka this period occurred during heinrich stadial 1 hs 1 when an extensive tropical megadrought has been postulated stager et al 2011 heinrich stadial 1 is represented by two episodes of gypsum precipitation and high 18o and 13c values in peten itza interrupted by an intervening period of lower 18o and 13c and clay deposition centered on 17 cal ka bp the two periods of inferred maximum cold and or arid conditions at 17 5 and 16 1 cal ka bp coincide approximately with two pulses of ice rafted debris ird recorded off southern portugal bard et al 2000 at 15 cal ka bp coinciding with the start of the bolling allerod period 18o and 13c decrease and gypsum precipitation ceases indicating a transition to warmer and or wetter conditions gypsum precipitation resumed while 18o and 13c increased at the start of the younger dryas at 13 1 cal ka bp and continued until 10 4 cal ka bp near the onset of the holocene precipitation changes during the last glacial period in the northern hemisphere neotropics were closely linked with freshwater forcing to the high latitude north atlantic and sensitive to changes in the location of meltwater input climate was coldest driest when meltwater directly entered the high latitude north atlantic permitting sea ice expansion and weakening of atlantic meridional overturning circulation amoc which resulted in a more southerly position of the intertropical convergence zone itcz upon deglaciation when meltwater was directed to the gulf of mexico at 17 ka and during the bolling allerod period 15 13 ka precipitation increased in the northern hemisphere neotropics as north atlantic sea ice retreated and the itcz shifted northward results from lake peten itza offer some support for the meltwater routing hypothesis of clark et al 2001

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

a 25 000 year tropical climate history from bolivian ice cores ice cores that were recovered from the summit of sajama mountain in bolivia provide carbon 14 dated tropical records and extend to the late glacial stage lgs oxygen isotopic ratios of the ice decreased 5 4 per mil between the early holocene and the last glacial maximum which is consistent with values from other ice cores the abrupt onset and termination of a younger dryas type event suggest atmospheric processes as the probable drivers regional accumulation increased during the lgs during deglaciation and over the past 3000 years which is concurrent with higher water levels in regional paleolakes unlike polar cores sajama glacial ice contains eight times less dust than the holocene ice which reflects wetter conditions and extensive snow cover

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

paleoecology of the guayana highlands northern south america holocene pollen record from the eruoda tepui in the chimanta massif the previously recorded vegetation constancy during most of the holocene atop some summits of the guayana tabular mountains or tepuis led to the hypotheses of either environmental stability or site insensitivity as high mountain biomes are considered to be especially well suited for recording past environmental changes a palynological study on the uppermost summit of the chimanta massif was designed to test its suitability for these purposes a peat sequence was obtained spanning the last 13 0 cal kyr bp but an acceptable resolution for paleoecological reconstruction is available only for the last 4000 years around 4 3 cal kyr bp the modern vegetation was established and has remained virtually unchanged until today minor paleoenvironmental changes recorded in other sequences around 2 5 cal kyr bp were not detected here the main paleoclimatic trends are in good agreement with other neotropical records especially from lake valencia and the cariaco basin it is concluded that high altitude tepuian sites are useful to record paleoenvironmental changes of moderate to high intensity but once a dense vegetation cover is established gentle shifts remain hidden due to the capacity of plant communities to absorb the changes the best sites for paleoecological research atop the tepuis are those lying on or near altitudinal ecotones especially between the meadows and the paramoid shrublands 2200 m elevation sites within the meadow domain as most well studied so far are relatively insensitive to holocene paleoenvironmental changes

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

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

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

glacial and postglacial pollen records from the ecuadorian andes and amazon abstract a long pollen record is derived from sediments of a lake dammed behind a low moraine of the last glaciation at 3 s latitude in the ecuadorian andes and is compared with a glacial age pollen record from the amazon rainforest immediately below lake surucucho llaviucu lies at 3180 m on the amazonian flank of the andes and above the glacial age pollen record from san juan bosco at 970 m the surucucho pollen record is interpreted as showing treeless vegetation in glacial times advance of treeline in late glacial time and holocene development of modern andean forests combining the surucucho and san juan bosco records shows that andean vegetation was affected by glacial cooling at all elevations vegetation did not move up and down slope as belts rather plant associations were reformed as temperature sensitive species found different centers of distribution with changing temperature

amazon diversity in light of the paleoecological record abstract disjunct distributions of amazonian species have been explained previously by a refugial theory which postulates that amazonian rain forest was preserved in large highland regions throughout the pleistocene no direct radiocarbon dated evidence exists for the last glacial maximum with which to test this theory the only radiocarbon dates of pleistocene age from the amazon basin are of fossiliferous deposits in the proposed napo refugium of the west where both pollen assemblages and wood samples indicate that forest with cool andean elements existed there at two intervals in the last cycle of northern hemisphere glaciation implying a temperature depression of at least 4 c in the amazon lowlands under modern climatic conditions lateral erosion by river meander together with surface erosion serves as a rejuvenating mechanism for the rain forests of peru and ecuador the instability of late holocene amazonian climates is demonstrated by documenting a precipitation event in the eastern andean cordillera that caused widespread flooding of western amazonian forests 800 1300 bp late holocene pollen histories from widely dispersed parts of central amazonia distinguish between vegetation histories in the drainage of northern south western and western watersheds but all show histories of fluctuating intensities of dry seasons radiocarbon dating of charcoal layers in soils of venezuelan amazonia demonstrates the apparently random incidence of wild fires at wide intervals over at least the last 6 ka the high species richness of amazonia is a result of numerous opportunities for vicariance because of a very large total area wide variety of habitats and intermediate levels of disturbance particularly by hydrological processes that has varied on timescales from years to millennia amazonian disjunct distributions probably reflect regional environmental discontinuities in both interglacial and glacial times

physiological significance of low atmospheric co2 for plant climate interactions abstract methods of palaeoclimate reconstruction from pollen are built upon the assumption that plant climate interactions remain the same through time or that these interactions are independent of changes in atmospheric co 2 the latter may be problematic because air trapped in polar ice caps indicates that atmospheric co 2 has fluctuated significantly over at least the past 400 000 yr and likely the last 1 6 million yr three other points indicate potential biases for vegetation based climate proxies first c 3 plant physiological research shows that the processes that determine growth optima in plants photosynthesis mitochondrial respiration photorespiration are all highly co 2 dependent and thus were likely affected by the lower co 2 levels of the last glacial maximum second the ratio of carbon assimilation per unit transpiration called water use efficiency is sensitive to changes in atmospheric co 2 through effects on stomatal conductance and may have altered c 3 plant responses to drought third leaf gas exchange experiments indicate that the response of plants to carbon depleting environmental stresses are strengthened under low co 2 relative to today this paper reviews the scope of research addressing the consequences of low atmospheric co 2 for plant and ecosystem processes and highlights why consideration of the physiological effects of low atmospheric co 2 on plant function is recommended for any future refinements to pollen based palaeoclimatic reconstructions

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

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

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

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

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

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

an 85 ka record of climate change in lowland central america drill cores obtained from lake peten itza peten guatemala contain a 85 kyr record of terrestrial climate from lowland central america that was used to reconstruct hydrologic changes in the northern neotropics during the last glaciation sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions respectively from 85 to 48 ka sediments were dominated by carbonate clay indicating moist conditions during marine isotope stages mis 5a 4 and early 3 the first gypsum layer was deposited at 48 ka signifying a shift toward drier hydrologic conditions and the onset of wet dry oscillations during the latter part of mis 3 peten climate varied between wetter conditions during interstadials and drier states during stadials the pattern of clay gypsum wet dry oscillations during the latter part of mis 3 48 23 ka closely resembles the temperature records from greenland ice cores and north atlantic marine sediment cores and precipitation proxies from the cariaco basin the most arid periods coincided with heinrich events when cold sea surface temperatures prevailed in the north atlantic meridional overturning circulation was reduced and the intertropical convergence zone itcz was displaced southward a thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake and pollen accumulated during the same period indicates vegetation consisted of a temperate pine oak forest this finding contradicts previous inferences that climate was arid during the last glacial maximum lgm chronozone 21 2 ka at 18 ka peten climate switched from moist to arid conditions and remained dry from 18 to 14 7 ka during the early deglaciation moister conditions prevailed during the warmer bolling allerod 14 7 12 8 ka with the exception of a brief return to dry conditions at 13 8 ka that coincides with the older dryas and meltwater pulse 1a the onset of the younger dryas at 12 8 ka marked the return of gypsum and hence dry conditions the lake continued to precipitate gypsum until 10 3 ka when rainfall increased markedly in the early holocene

constancy in the vegetation of the amazon basin during the late pleistocene evidence from the organic matter composition of amazon deep sea fan sediments analyses of more than 60 sediment samples from the amazon deep sea fan show remarkably constant terrigenous biomarkers lignin phenols and cutin acids and stable carbon isotopic compositions of organic matter batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document 13 c om end document deposited from 10 to 70 ka sediments from the nine amazon deep sea fan channel levee systems investigated in this study yielded relatively narrow ranges for diagnostic parameters such as organic carbon oc normalized total lignin yields 3 1 1 1 mg 100 mg oc syringyl vanillyl phenol ratios s v 0 84 0 06 cinnamyl vanillyl phenol ratios c v 0 08 0 02 isomeric abundances of cutin derived dihydroxyhexadecanoic acid f10 16 oh 0 65 0 02 and batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document 13 c om end document 27 6 0 6 our measurements support the hypothesis that the vegetation of the amazon basin did not change significantly during the late pleistocene even during the last glacial maximum moreover the compositions obtained from the amazon deep sea fan are similar to those of modern amazon river suspended sediments such results strongly indicate that the current tropical rainforest vegetation has been a permanent and dominant feature of the amazon river watershed over the past 70 k y specifically we found no evidence for the development of large savannas that had been previously postulated as indicators of increased glacial aridity in amazonia climate models need to be modified to account for the uninterrupted input of moisture to the tropical amazon region over the late pleistocene holocene period

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

impact of mid to late holocene precipitation changes on vegetation across lowland tropical south america a paleo data synthesis a multi proxy paleo data synthesis of 110 sites is presented exploring the impact of mid to late holocene precipitation changes upon vegetation across southern hemisphere tropical south america we show that the most significant vegetation changes occurred in southwest amazonia and southeast brazil regions reliant on precipitation derived from the south american summer monsoon sasm a drier mid holocene in these regions linked to a weaker sasm favored more open vegetation savannah grasslands than present while increased late holocene precipitation drove expansion of humid forests e g evergreen tropical forest in southwest amazonia araucaria forests in southeast brazil the tropical forests of central western and eastern amazonia remained largely intact throughout this 6000 year period northeastern brazil s climate is antiphased with the rest of tropical south america but a lack of paleo data limits our understanding of how vegetation responded to a wetter drier mid late holocene from this paleo data perspective we conclude that ecotonal forests already close to their climatic thresholds are most vulnerable to predicted future drought but the forest biome in the core of amazonia is likely to be more resilient of greater concern are widespread deforestation and uncontrolled anthropogenic burning which will decrease ecosystem resilience making them more vulnerable than they might be without current anthropogenic pressures

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

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

the evolution of a tropical rainforest grassland mosaic in southeastern brazil since 28 000 14c yr bp based on carbon isotopes and pollen records the lack of paleoecological records from the montane atlantic rainforest of coastal brazil a hotspot of biological diversity has been a major obstacle to our understanding of the vegetational changes since the last glacial cycle we present carbon isotope and pollen records to assess the impact of the glaciation on the native vegetation of the serra do mar rainforest in sao paulo brazil from ca 28 000 to 22 000 14c yr bp a subtropical forest with conifer trees is indicative of cool and humid conditions in agreement carbon isotopic data on soil organic matter suggest the presence of c3 plants and perhaps c4 plants from 28 000 to 19 000 14c yr bp the significant increase in the sedimentation rate and algal spores from 19 450 to 19 000 14c yr bp indicates increasing humidity associated to an erosion process between 19 000 and 15 600 14c yr bp from 15 600 14c yr bp to present there is a substantial increase in arboreal elements and herbs indicating more humid and warmer climate from 19 000 to 1000 14c yr bp 13c values indicated the predominance of c3 plants these results are in agreement with studies in speleothems of caves which suggest humid conditions during the last glacial maximum

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

pleistocene precipitation balance in the amazon basin recorded in deep sea sediments abstract terrigenous sediments from ceara rise in the western tropical atlantic ocean record pleistocene amazon basin climate variability iron oxides and oxyhydroxides in this region originate mainly from chemically leached amazon lowland soils concentrations of goethite and hematite in the terrigenous fraction consistently peak during transitions from glacial to interglacial periods suggesting an increased proportion of erosive products derived from the amazon lowlands compared to the physically weathered highlands lowland amazon basin precipitation changes monitored by the percentage of goethite relative to total iron oxides lead ice age extremes with maximum aridity during ice growth and maximum precipitation during ice melt rapid climate changes over the amazon basin may reflect shifts in the position of the intertropical convergence zone forced by northern hemisphere insolation at precessional 1 23 000 yr 1 and obliquity 1 41 000 yr 1 frequencies variance in the orbital eccentricity bands 1 100 000 and 1 413 000 yr 1 may be explained by nonlinear amplification of insolation forcing at precessional frequencies the early response of amazon precipitation to insolation ahead of high latitude ice volume 18o at all orbital frequencies suggests that tropical aridity is part of the chain of events leading to ice ages rather than a response to glacier oscillations

foraminiferal faunal estimates of paleotemperature circumventing the no analog problem yields cool ice age tropics the sensitivity of the tropics to climate change particularly the amplitude of glacial to interglacial changes in sea surface temperature sst is one of the great controversies in paleoclimatology here we reassess faunal estimates of ice age ssts focusing on the problem of no analog planktonic foraminiferal assemblages in the equatorial oceans that confounds both classical transfer function and modern analog methods a new calibration strategy developed here which uses past variability of species to define robust faunal assemblages solves the no analog problem and reveals ice age cooling of 5 o to 6oc in the equatorial current systems of the atlantic and eastern pacific oceans classical transfer functions underestimated temperature changes in some areas of the tropical oceans because core top assemblages misrepresented the ice age faunal assemblages our finding is consistent with some geochemical estimates and model predictions of greater ice age cooling in the tropics than was inferred by climate long range investigation mapping and prediction climap 1981 and thus may help to resolve a long standing controversy our new foraminiferal transfer function suggests that such cooling was limited to the equatorial current systems however and supports climap s inference of stability of the subtropical gyre centers

contrasting pollen histories of mis 5e and the holocene from lake titicaca bolivia peru two long sediment records cores lto1 2b and lt01 3b from lake titicaca bolivia peru are compared with a previously analysed holocene record from this lake core ne98 1pc the holocene records of lt01 2b and ne98 1pc are similar there are striking differences however between the mis 5e sections of the long cores and the holocene records in these records tempera ture is probably the dominant parameter that determines the total fossil pollen concentration and is used to time the onset and termination of deglaciation in contrast the relative and absolute abun dance of specific taxa e g polylepis acaena chenopodiaceae are indicators of relative moisture availability although the holocene contains a period of aridity between ca 8000 cal yr bp and 4300 cal yr bp it is a minor event compared with the more extreme aridity of mis 5e core lt01 3b showed similar trends during mis 5e when compared to lt01 2b as did ne98 1pc when com paring holocene records mis 5e and the holocene are markedly different interglacials depicted by shifts in pollen concentration and taxa representation over time copyright 2005 john wiley sons ltd

a 24 700 yr paleolimnological history from the peruvian andes abstract a new paleolimnological dataset from lake pacucha 13 s 3095 m elevation in the peruvian andes provides evidence of changes in lake level over the past 24 700 yr a late glacial highstand in lake level gave way to an early holocene lowstand this transition appears to have paralleled precessional changes that would have reduced insolation during the wet season the occurrence of benthic salt tolerant diatoms and caco3 deposition suggest that the lake had lost much of its volume by c 10 000 cal yr bp pronounced holocene oscillations in lake level included a second phase of low lake level and heightened volatility lasting from c 8300 to 5000 cal yr bp while a polymictic lake formed at c 5000 cal yr bp these relatively wet conditions were interrupted by a series of drier events the most pronounced of which occurred at c 750 cal yr bp paleolimnological changes in the holocene were more rapid than those of either the last glacial maximum or the deglacial period

simulation of the holocene climate over south america and impacts on the vegetation we provide a comprehensive analysis of the holocene climate and vegetation changes over south america through numerical simulations holocene climate for several periods 8 ka 6 ka 4 ka 2 ka an

modern pollen spectra from costa rica abstract we analyzed pollen spectra preserved in surface lake and mire sediments and soils to provide information on modern pollen rain across a range of vegetation types in costa rica we collected samples at 29 sites located within ten principal vegetation types mangrove tropical dry forest derived savanna tropical moist forest tropical wet forest lowland freshwater swamp premontane rain forest montane rain forest montane bog and paramo samples from sites with similar vegetation generally had similar pollen assemblages and clustered together in detrended correspondence analysis results indicate that major vegetation types in costa rica have distinct pollen spectra

environmental changes in southeastern amazonia during the last 25 000 yr revealed from a paleoecological record abstract new pollen micro charcoal sediment and mineral analyses of a radiocarbon dated sediment core from the serra sul dos carajas southeast amazonia indicate changes between drier and wetter climatic conditions during the past 25 000 yr reflected by fire events expansion of savanna vegetation and no analog amazonian forest communities a cool and dry last glacial maximum lgm and late glacial were followed by a wet phase in the early holocene lasting for ca 1200 yr when tropical forest occurred under stable humid conditions subsequently an increasingly warm seasonal climate established the onset of seasonality falls within the early holocene warm period with possibly longer dry seasons from 10 200 to 3400 cal yr bp and an explicitly drier phase from 9000 to 3700 cal yr bp modern conditions with shorter dry seasons became established after 3400 cal yr bp taken together with paleoenvironmental evidence from elsewhere in the amazon basin the observed changes in late pleistocene and holocene vegetation in the serra sul dos carajas likely reflect large scale shifts in precipitation patterns driven by the latitudinal displacement of the inter tropical convergence zone and changes in sea surface temperatures in the tropical atlantic