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

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

amazon fan biomarker evidence against the pleistocene rainforest refuge hypothesis ocean drilling program leg 155 site 942 on the amazon fan is an ideal location for monitoring palaeoclimatic changes within a significant proportion of the amazon basin we present n alkane d 13 c and taraxerol and laevoglucosan concentration records from this site covering the last 38 ka the entire n alkane d 13 c record is constrained between 31 and 34 which is well within the isotopic range occupied by c3 vegetation the concentration and relative abundance of taraxerol a mangrove indicator varies by over an order of magnitude but seems to have had no effect on the n alkane d 13 c record the laevoglucosan concentrations are extremely low during the last glacial period suggesting a relatively low occurrence of forest fires laevoglucosan concentrations are highest between 13 5 and 12 5 ka suggesting an increased incidence of amazon forest fires at the very end of the younger dryas these records combined with previously published pollen records from site 932 reveal no evidence for massive incursions of grasslands into amazonia during the last glacial period despite evidence of reduced outflow of the amazon river indicating more arid conditions we therefore suggest that savannah encroachment as proposed by the pleistocene refuge hypothesis can be refuted as an explanation for high species endemism within the amazon basin and alternative explanations are required copyright 2012 john wiley sons ltd

late quaternary paleoenvironmental studies in the gran sabana venezuelan guayana shield abstract previous work in northern south america has suggested that during the late pleistocene glaciation this region had a drier climate than the present one this led to the postulation of the refuge hypothesis the gran sabana has been postulated as part of the pleistocene pantepui refuge radiocarbon dating of peat deposits on the table mountain summits and in the surrounding lowlands suggests that the peat formation began in the late pleistocene early holocene and continues to the present alluvial sediments on the table mountain piedmonts cut into terrace complexes suggest that they were accumulated by rapid torrential chaotic and intermittent events typical of climates with pronounced dry seasons these sediments were assigned pleistocene to holocene relative ages these data to not support the existence of the pantepui refuge during the late pleistocene

late quaternary vegetation fire and climate history reconstructed from two cores at cerro toledo podocarpus national park southeastern ecuadorian andes the last ca 20 000 yr of palaeoenvironmental conditions in podocarpus national park in the southeastern ecuadorian andes have been reconstructed from two pollen records from cerro toledo 04 22 28 6 s 79 06 41 5 w at 3150 m and 3110 m elevation paramo vegetation with high proportions of plantago rigida characterised the last glacial maximum lgm reflecting cold and wet conditions the upper forest line was at markedly lower elevations than present after ca 16 200 cal yr bp paramo vegetation decreased slightly while mountain rainforest developed suggesting rising temperatures the trend of increasing temperatures and mountain rainforest expansion continued until ca 8500 cal yr bp while highest temperatures probably occurred from 9300 to 8500 cal yr bp from ca 8500 cal yr bp paramo vegetation re expanded with dominance of poaceae suggesting a change to cooler conditions during the late holocene after ca 1800 cal yr bp a decrease in paramo indicates a change to warmer conditions anthropogenic impact near the study site is indicated for times after 2300 cal yr bp the regional environmental history indicates that through time the eastern andean cordillera in south ecuador was influenced by eastern amazonian climates rather than western pacific climates

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

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

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

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

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

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

the mera lahar deposit in the upper amazon basin transformation of a late pleistocene collapse at huisla volcano central ecuador abstract the sub andean zone east of the cordillera real ecuador and out to the amazon basin s western margin has been the depository of voluminous lahars related to volcanic activity in the andean highlands these lahars arrived to the sub andean zone via gravitational transport through narrow river canyons and emplaced volumes surpassing several cubic kilometers over robust inundation zones this paper discusses the origin flow route depositional zone terrace formation and geomorphic significance of the most important lahar deposit yet mapped in this region that of the mera lahar which likely formed from a late pleistocene collapse of huisla volcano followed by impounding by temporary dams behind drainage blocking debris avalanche deposits dad then subsequent rupture of the blockage the actual deposit of the mera lahar has a thickness between 30 and 70 m is mainly comprised of dad breccia is matrix supported c 70 of reddish gray color and is well consolidated based on geochemical and petrographic similarities huisla volcano s dad is the best candidate for the lahar s source huisla volcano is located some 90 km up valley of the bulk of mera lahar s mapped deposit clasts of mera lahar rocks and huisla s dad breccias have 57 61 wt sio2 corresponding to andesites of the calc alkaline series with mean values of 1 1 5 wt for k2o the mapped mera lahar deposit has an actual volume estimated in 1 2 km3 compared to its original estimated volume of 5 4 km3 cross sectional widths of 1 5 to 4 5 km span and extend laterally beyond the actual pastaza river channel where the lahar s deposition produced high standing isolated surfaces that are notable local geomorphic features of the upper sub andean zone the flow modeling program laharflow employing the modern landscape as the topographic base adequately simulates the flow route and inundation zones of the mera lahar

palaeoclimates in central and south america since 18 000 bp based on pollen and lake level records abstract the last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from central and south america a total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis 8 records from tropical and subtropical lowlands in central and south america 7 records from the central american highlands 26 records from the high andes north and south of the equator 2 records from the southern subtropics and 18 from the southern andes and subantarctic islands palaeoclimatic interpretation of the pollen records is semiquantitative based on bioclimatic parameters such as e g elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation additional palaeoclimatic information is obtained from lake level records derived either from dated shorelines or from the limnological history of lake sediment sections large scale palaeoclimatic comparison of the proxy data records indicates that full and late glacial climates throughout central and south america were cooler than today by about 4 to 5 c but that moisture patterns showed regional differentiation thus in both the southern subtropics and high latitudes in southern chile conditions were substantially moister than today during full and late glacial times whereas everywhere else conditions were far drier than today in terms of palaeocirculation this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation a scenario that has been suggested by palaeoclimatic simulations from general circulation models gcm another major palaeoclimatic aspect predicted by the gcm simulations is the suppressed early holocene summer monsoon in the southern subtropics although proxy records are few from this zone their palaeoenvironmental history during that time shows a period of aridity that would support the model one of the most controversial aspects of south american palaeoclimates concerns the existence of a younger dryas equivalent a cold period between 11 and 10 ka bp based on the existing data from south america that show a great diversity of palaeoclimatic trends the concept of an interval that is characterized by the same climatic signal as documented for the north atlantic seems too simplistic moreover at this point only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question the principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present and the realization of regions and periods where much more and concerted effort is needed to address problems of intra and interhemispheric palaeoclimatic correlations only then can questions concerning the mechanism of climatic change be tackled successfully

paleoecological record of hurricane disturbance and forest regeneration in nicaragua studying infrequent phenomena e g hurricanes and slow processes e g forest regeneration greatly challenges the ecological techniques of real time studies by combining the two relatively new approaches of paleotempestology and fine resolution palynology this study provides insight into the impacts of hurricanes and the post hurricane regeneration of forests i analyzed a 5 m sediment core from a swamp lagoon on the caribbean coast of nicaragua that covered the entire 8000 yr history of the swamp urquhart g r 1997 disturbance and regeneration of swamp forests in nicaragua evidence from ecology and paleoecology ph d dissertation the university of michigan ann arbor mi usa x rays revealed a sand layer dating to c 3300 bp of the type deposited by hurricanes pollen analyses showed this sand layer was followed by major changes in vegetation and fires this pattern is identical to the wake of hurricane joan which struck the area in 1988 and left 90 000 hectares of damaged swamp forest that burned shortly after after the prehistoric hurricane forest vegetation did not return until 500 yr later due to repeated burning this parallel event of the past illustrates a possible course for modern forest regeneration as a counterpart to direct ecological analysis fine resolution paleoecological study can provide great insight for the study of rare events and slow processes

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

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

nature of environmental changes in south america at the last glacial maximum abstract a review is given of geomorphological evidence suggesting the nature of environmental changes in south america during the last glaciation maximum lgm the data are from glacial geocryogenic alluvial colluvial and aeolian systems and span 68 of latitude making the most complete land transect of major climatic systems in the southern hemisphere glaciers in the northern and central andes may have reached their maximal extent by 27 000 yr b p reduced precipitation at the lgm caused by lower temperatures and lower atmospheric humidity probably led to slight glacier recession in the tropical andes icefields in the southern andes were most expansive when global temperature and sea level were lowest because pacific westerlies supplied abundant moisture and western outlets advancing across the continental shelf were constrained by sea level low gradient river networks became incised by at least 100 m into narrow channels as global sea level fell by 120 m during the lgm draw down of water tables possibly impacted the forest cover enhancing the drying influence of reduced sea surface temperature and atmospheric humidity as forest and grass covers diminished in extent effecting greater atmospheric cooling because of reduced evapotranspiration and convective condensation colluvial and aeolian processes became more active and widespread almost 25 of the continent is covered with palaeo aeolian features presumed to have been active when airflow was stronger due to steeper atmospheric pressure and temperature gradients at the lgm substantial changes in precipitation totals and incidence and in ground water availability probably eliminated modern type tropical rainforest except in areas currently receiving ca 5000 mm of annual rainfall equivalent changes in vegetation cover spread across much of the guyana and brazilian highlands and throughout the southern temperate regions

landscape scale drivers of glacial ecosystem change in the montane forests of the eastern andean flank ecuador understanding the impact of landscape scale disturbance events during the last glacial period is vital in accurately reconstructing the ecosystem dynamics of montane environments here a sedimentary succession from the tropical montane cloud forest of the eastern andean flank of ecuador provides evidence of the role of non climate drivers of vegetation change volcanic events fire regime and herbivory during the late pleistocene multiproxy analysis pollen non pollen palynomorphs charcoal geochemistry and carbon content of the sediments radiocarbon dated to ca 45 42 ka provide a snap shot of the depositional environment vegetation community and non climate drivers of ecosystem dynamics the geomorphology of the vinillos study area along with the organic carbon content and aquatic remains suggest deposition took place near a valley floor in a swamp or shallow water environment the pollen assemblage initially composed primarily of herbaceous types poaceae asteraceae solanaceae is replaced by assemblages characterised by andean forest taxa first melastomataceae weinmannia ilex and later alnus hedyosmum myrica the pollen assemblages have no modern analogues in the tropical montane cloud forest of ecuador high micro charcoal and rare macro charcoal abundances co occur with volcanic tephra deposits suggesting transportation from extra local regions and that volcanic eruptions were an important source of ignition in the wider glacial landscape the presence of the coprophilous fungi sporormiella reveals the occurrence of herbivores in the glacial montane forest landscape pollen analysis indicates a stable regional vegetation community with changes in vegetation population co varying with large volcanic tephra deposits suggesting that the structure of glacial vegetation community at vinillos was driven by volcanic activity

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

a multiproxy palaeoecological record of holocene lake sediments from the rio tapajos eastern amazonia abstract two sediment cores up to 42 m in length were raised from the wide deep section of the lower tapajos river amazonia referred to as lago tapajos these cores reveal a history of subtle environmental change that began with the formation of lago tapajos as sea level rose about 11 000 years ago the sediments of the lake were deposited fairly quickly at a rate of ca 4 m per millennium and are uniformly fine grained with low organic content the fossil pollen record derived from these sediments reveals that forest surrounded this site throughout the holocene the largest change in the core took place between ca 5500 and 4200 cal years bp and reflects a transition from coarser to finer sediments coincident with the change in sediment is a slight transition in the pollen spectra with an increase in poaceae abundance at the expense of the pioneer tree cecropia cecropia a tentative explanation is offered in which increased human activity possibly spurred by climatic change resulted in the formation of some local grasslands however despite the apparent actions of humans there is no indication of basin wide transformation of landscapes in this record the holocene persistence of forest as the dominant landscape matrix around lago tapajos is supported by low 13 c values and by the constant geochemistry and mineralogy of the lake sediments

gold bearing iron formation in a granulite terrane of the canadian shield a possible deep level expression of an archean gold mineralizing system gold occurs in metamorphosed iron formation within a migmatized paragneiss terrane in the eastern superior province of the canadian shield the main gold prospect which is located 100 m from a tonalite pluton has undergone skarn like metamorphism that occurred late during the granulite grade metamorphism despite its occurrence in a high grade metamorphic host rock the mineralization shares many of the characteristics of lowergrade archean greenstone belt gold mineralization such as 1 enrichment in au as s sb w u and feo 2 a low base metal gold ratio 3 the inclusion of gold in arsenopyrite 4 the proximity to a felsic pluton 5 an iron rich fractured host rock and 6 a similar mineralization time around 2650 ma this gold occurrence occurs near the metamorphic peak and represents a deep level expression of the widespread crustal scale mineralizing system documented for the late archean the main conclusion of this work is that the principal characteristics of archean gold mineralization are present in the deep crust and do not come from the leaching of upper level rock piles such as greenstone belts

from oxisols to spodosols and histosols evolution of the soil mantles in the rio negro basin amazonia abstract the upper rio negro basin under a constantly humid equatorial climate is a low altitude peneplain of more than 165 000 km 2 prevailing soils are oxisols and spodosols and their distribution is usually related to the lithology of the parent materials spodosols being generally associated with sandy deposits after exploratory surveys in this extensive region six major soil geomorphic units have been identified in which selected toposequences have been studied by means of micromorphological chemical and mineralogical analyses detailed field analysis of the horizonation of the soil mantle has been carried out in three sequences consisting of oxisols ultisols and spodosols results show that sharp transitions within distances of less than one hundred meters separate the oxisols from the spodosols however the arrangement of the horizons in the soil mantle and similarities in micromorphological features chemical composition and mineral components between the adjacent horizons are evidences that genetic relationships link contiguous profiles the authors propose an alternative explanation for this soil distribution based on the lateral transformation of oxisols and ultisols into spodosols and on the lateral evolution of the giant spodosols into histosols peat and waterlogged ultisols interpretative models of landscape evolution as consequence of soil evolution are thus proposed

modern pollen vegetation relationships along a steep temperature gradient in the tropical andes of ecuador the characterization of modern pollen rain assemblages along environmental gradients is an essential prerequisite for reliable interpretations of fossil pollen records in this study we identify pollen vegetation relationships using modern pollen rain assemblages in moss polsters n 13 and lake sediment surface samples n 11 along a steep temperature gradient of 7 c 3100 4200 m above sea level on the western andean cordillera ecuador the pollen rain is correlated to vascular plant abundance data recorded in vegetation relevees n 13 results show that pollen spectra from both moss polsters and sediment surface samples reflect changes in species composition along the temperature gradient despite overrepresentation of upper montane forest taxa in the latter estimated pollen transport distance for a lake laguna llaviucu situated in a steep upper montane forest valley is 1 2 km while a lake laguna pallcacocha in the paramo captures pollen input from a distance of up to 10 40 km weinmannia spp podocarpus spp and hedyosmum sp are indicators of local upper montane forest vegetation while phlegmariurus spp and plantago spp are indicators for local paramo vegetation

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

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

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

the coolgardie goldfied western australia district scale controls on an archaean gold camp in an amphibolite facies terrane archaean lode gold deposits in the coolgardie goldfield western australia occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies mineralisation was broadly synchronous with peak metamorphism the main phase of granitoid emplacement and regional deformation several different structural styles are represented by the deposits of the coolgardie goldfield mineralisation occurs along sheared felsic porphyry ultramafic rock contacts in gabbro hosted quartz vein sets in fault bounded quartzvein sets and in laminated quartz reefs sited in brittleductile shear zones the structures hosting mineralisation formed in response to a progressive deformation event related to granitoid emplacement in an east west compressional far field stress regime but with local heterogeneous stress orientations this occurred after an earlier period of thrust stacking with probable north south directed tectonic transport two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the coolgardie goldfield a high temperature assemblage formation temperature 500 c characterised by the proximal alteration assemblage garnet hornblende plagioclase pyrrhotite contrasts with a medium temperature assemblage formation temperature 500 c consisting of calcic amphibole biotite plagioclase calcite arsenopyrite pyrrhotite the distribution of the two styles of gold related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the coolgardie goldfield with the high temperature style of alteration more proximal to the granitoid greenstone contact than the medium temperature style the occurrence of gold deposits that formed under amphibolite facies conditions throughout the coolgardie goldfield supports a crustal continuum model for archaean lode gold deposits which proposes that gold is deposited in metamorphic environments that range from the sub greenschist to granulite facies in addition the data from coolgardie suggest that syntectonic synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock alteration at deep crustal levels

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

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

triassic turbidites in the west qinling mountains nw china part of the collisional songpan ganzi basin or an active forearc basin abstract lower to middle triassic clastic rocks in the west qinling mountains along ne margin of the qinghai tibet plateau are generally regarded as part of the songpan ganzi flysch basin however relatively little attention has been paid to the age and provenance of these units new petrological and geochemical results demonstrate that these sediments accumulated along an active continental margin and are dominated by feldspathic litharenite and lithic arkose with low mineral and compositional maturity they were derived primarily from a continental arc source dominated by intermediate to felsic igneous rocks with a minor contribution from older metamorphosed and sedimentary sources turbidite samples yielded two primary detrital zircon u pb age populations of ca 273 ma and ca 435 ma which is different from the early triassic ca 252 ma and middle silurian ca 427 ma age populations that dominate the songpan ganzi basin these data together with paleocurrent results indicate that the south qilian belt was the primary source origin because this belt contains both early paleozoic andean type igneous and magmatic rocks and has a basement of precambrian metamorphosed rocks regionally an andean type arc traverses the south qilian belts and extends into the west qinling mountains and kunlun orogen which formed by north directed subduction of the paleo tethyan ocean during the early to middle triassic voluminous detritus that originated from the qilian and kunlun orogens as they were uplifted and eroded was transported to the south and deposited in the forearc area in front of the a nimaqen mianlue suture

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

environmental processes of the ice age land oceans glaciers epilog knowledge of the state of the earth at the last glacial maximum lgm an interval around 21 000 years ago is an important benchmark for understanding the sensitivity of global environmental systems to change much progress in understanding climates of the lgm has occurred in the 20 years since the end of the climap project of the 1970s climate long range investigation mapping and prediction here we review this progress based on presentations and discussion at a rst open science meeting of the epilog project environmental processes of the ice age land oceans glaciers held in delmenhorst germany may 1999 we outline key controversies and document protocols for epilog contributions so that a new synthesis of the lgm earth can emerge as an open project of the world s community of scientists 2001 elsevier science ltd all rights reserved

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

alluvial lithofacies recognition in a humid tropical setting abstract cobble gravel deposits in the antigua formation accumulated on a large alluvial fan or braid plain west of the cordillera occidental in southwest colombia this formation was probably deposited during the pleistocene in a very wet tropical climate 500 cm yr rainfall fining upwards sequences of clast supported imbricated boulders and cobbles dominate with maximum clast sizes between 30 and 300 cm the sand matrix in the antigua gravels and the minor 10 sand facies are weathered to clay at depths of up to 20 m the sand facies contains abundant drift logs and leaf mats except for the absence of debris flows and the very coarse nature of the gravel the antigua gravels have lithofacies similar to the glacial outwash braid plain in the proximal area of the scott type model gravels and sands of the younger panambi formation were deposited by a braided stream that was smaller confined by valley walls and flowing at a lower gradient than the river that deposited the antigua gravels we recognize no sedimentologic characteristics of these deposits as diagnostic of a humid tropical environment except for textural and compositional changes in matrix sediments caused by deep and rapid chemical weathering

colombian dry moist forest transitions in the llanos orientales a comparison of model and pollen based biome reconstructions colombian vegetation at the ecological level of the biome is reconstructed at six sites using pollen data assigned a priori to plant functional types and biomes the chosen sites incorporate four savanna sites laguna sardinas laguna angel el pinal and laguna carimagua a site on the transition between savanna and amazon rainforest loma linda and a site within the amazon rainforest pantano de monica the areal extent of tropical moist forest tropical dry forest and steppe have been subject to significant change differential responses of the vegetation to climatic shifts are related to changes in plant available moisture duration of dry season and edaphic controls on the vegetation the record from el pinal shows that the present day savanna vegetation dominated by steppe poaceae with little occurrence of woody savanna taxa e g curatella byrsonima was present since the last glacial period of the northern hemisphere unfortunately el pinal is located on an edaphic savanna and is not particularly responsive to registering change most records cover the early holocene one site records the el abra stadial younger dryas equivalent when forest expansion reflects more humid climatic conditions and higher plant available moisture during the early and middle holocene the maximum expansion of steppe and tropical dry forest occurred indicating that dry climatic conditions continued to around 4000 14 c bp the following period from shortly before 4000 14 c bp is characterised by an increase in forest and gallery forests reflecting a wetter period probably with a shorter annual dry season anthropogenic influence on the vegetation is recorded by all the records over the last millennial particularly characterised by a reduction in forest cover and high amplitude changes in vegetation biome transitions from one type to another and the environmental controls on this shift are investigated by applying a vegetation model biome 3 the model uses climatic data from six meteorological stations that encompass a range of environments within lowland colombia which are similar to the pollen data the signals of vegetation change can be translated to the main environmental controls of temperature and moisture to indicate the degree of change needed in these parameters to record the vegetation change depicted by the pollen data moisture balance is the dominant control on driving vegetation change whether under seasonal or annual control the combined reconstruction from pollen data and model output of biome scale vegetation dynamics for lowland colombia allows an understanding of the environmental controls to be developed d 2005 elsevier b v all rights reserved

synmetamorphic lode gold deposits in high grade archean settings lode gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the yilgarn block of western australia these deposits which form a continuum with lower metamorphic grade mesothermal gold depos its show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust studies of 18 o suggest that fluid compositions were controlled by the wall rocks at least for some elements these more proximal gold deposits provide more information about deep fluid sources than do investigations of higher level mineralization deposited from fluids strongly modified by wall rock interaction during the early part of their ascent

geomorphic evidence of active deformation and uplift in a modern continental wedge top foredeep transition example of the eastern ecuadorian andes abstract the eastern ecuadorian andes appear as a fold and thrust belt adjacent to a continental foredeep represented by one of the world s largest tropical alluvial megafans the pastaza megafan debouching into the amazonian lowland the apex of the pliocene pleistocene megafan situated in the present day wedge top subandean zone has been cut by an erosion surface the western part of which has been uplifted of 500 m along the frontal thrust forming a poorly dissected plateau the mera plateau this erosion surface erased most of the previous fluvial landscape but preserved a large thrust related anticlinal hinge deforming less erodible underlying strata the mirador fold and smaller sized anticlines this surface has been then incised by two antecedent major rivers the pastaza and the napo and few tributaries the plateau edge is marked by a series of large scale gently sloping landslides clustered along a 70 km long concave eastward line associated with the frontal thrust fault the newly formed immature rivers issued from the landslides or sourced within east dipping remnants of the erosion surface downstream of the landslide line constitute the greatest part of the streams feeding the ecuadorian amazonian basin at 70 to 100 km from the landslide line the drainage abruptly changes from highly immature to mature with a well defined hinge line representing the outer limit of landslide and tectonic control the diversions of the pastaza river indicate ongoing fold growth since at least the late pleistocene in the eastern cordillera and the early holocene in the mera plateau the preserved terraces of the pastaza valley are all degradational and are ascribed to periods of tectonic seismic activity alternating with periods of tectonic quiescence or decreased seismic activity rather than to climatic events 14c dating of the plateau erosion surface and of the upper pastaza terraces indicates that the minimum average incision rate since 18 000 years bp varies locally in the upper pastaza valley from 0 5 to 0 67 cm year 1 increasing from 18 000 years bp to now a comparison of these incision rates with fold and thrust fault uplift rates indicates that incision in the upper pastaza valley was a result of rapid uplift up to 1 cm year 1 along the mirador fold and thrust which caused a restoration of the local equilibrium profile of the upper reach combined with smaller local fault uplift along the westernmost thrust faults the uplift of the whole mera plateau with respect to the upper amazonian basin gives a minimum average uplift rate of 2 8 cm year 1 since 18 000 years bp the overall uplift of the mera plateau and the eastern cordillera is likely to have been caused by a regional scale low angle thrust ramp emerging as the frontal thrust fault

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

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

late quaternary climate history of the bolivian altiplano abstract sediment cores and outcrops from the bolivian altiplano are used to interpret late pleistocene holocene paleoclimates and lake evolution based on a multi proxy approach ostracod content palynology sedimentology and radiocarbon dating despite the different sensitivity of the lacustrine basins to environmental changes interpreting the records in terms of paleohydrology climate and more especially timing of events is difficult notwithstanding these problems lake titicaca in the north and lake pocoyu lake poopo salars of coipasa and uyuni in the south reveal a similar evolution in general trends through the course of time during the minchin phase until 30 26 kyr bp the altiplano was wetter than present the last glacial maximum ca 26 14 kyr bp marked by cooler drier conditions was followed by a return to a wetter climate interrupted by short arid events between ca 14 and 10 5 kyr bp another cycle of aridity between 10 5 and 8 kyr bp took place abruptly just after the humid tauca phase the mid holocene is interpreted as climatically unstable with an alternation of humid and dry episodes since 3 9 kyr bp wetter conditions have persisted and intensified to the present the data are interpreted in terms of changes in inter tropical convergence zone extension summer precipitation and in polar air mass intensity winter precipitation

paleoenvironments and human occupation in late glacial panama the first pollen and phytolith data covering the entire pleistocene holocene transition from the lowlands of the central american isthmus indicate that the forests of late glacial panama at an altitude of 650 m resembled those currently found at ca 1500 1800 m a temperature depression of ca 5 c and reduced precipitation evaporation ratios in the late glacial period are suggested forest composition from ca 14 000 to 10 500 yr b p although primarily montane in character contained a low biomass of species today segregated in lowland forests and hence a floral assemblage with no modern analog the sudden appearance of carbon and burnt weedy plant material at ca 11 000 yr b p is attributed to the earliest human impact yet recorded from tropical america and may perhaps have been associated with the first human occupation of the region

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

evidence for a prolonged retroflection of the north brazil current during glacial stages the importance of the role played by the tropics in driving and propagating climate change between hemispheres has long been the focus of attention in a bid to evaluate ocean atmosphere interactions on glacial interglacial timescales the amazon fan and ceara rise in the western equatorial atlantic ocean lie directly in the flowpath of the north brazil current nbc which as a conduit for the cross equatorial transport of heat and salinity is a key component for the heat budget of the north atlantic mg ca palaeothermometry and stable oxygen isotope analysis of planktonic foraminifera sampled from 15 sites across the amazon fan and ceara rise reveal variations in oceanic surface currents and of the dispersal of freshwater from the river amazon over five timeslices modern early holocene younger dryas late glacial and marine isotope stage 3 sea surface temperature reconstructions reveal progressive climatic amelioration over the last similar to 30 ka indicating a temperature increase of similar to 3 2 1 1 degrees c since the late glacial in conjunction with this warming values of delta delta o 18 a proxy for water column stratification indicates increased vertical mixing of the glacial ocean the spatial distribution of values of delta 18 w the isotopic composition of ambient seawater is used to infer surface current variations and demonstrates an oceanward shift in the river outflow plume in cold climates representing a prolongation or possibly permanent continuation in the duration of the seasonal retroflection of the nbc causing the curtailment of cross equatorial transport a prolongation of this retroflection could have resulted from a mean southward migration of the southern boundary of the itcz c 2011 elsevier b v all rights reserved

a classification scheme for epigenetic archaean lode gold deposits recent research has shown that archaean lode gold deposits occur in environments that range in their metamorphic grade from prehnite pumpellyite to lower granulite facies based on this data a new classification is proposed for these deposits in this classification shallow level gold deposits are classed as epizonal the so called mesothermal deposits are mesozonal and the deeper deposits commonly in mid amphibolite or lower granulite terrains are hypozonal

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

full glacial summer temperatures in eastern north america as inferred from wisconsinan vegetational zonation abstract a variety of types of fossil evidence pollen macrofossils and snails document the occurrence of boreal forest species at low latitudes of eastern north america duringthe last full glacial 18 000 yr b p both the northern and southern boundaries of boreal coniferous forest are determined by limiting values of summer warmth knowledge of these limiting factors permits construction of a gradient of wisconsinan summer temperatures the reconstructed temperature departures are 5 12 c for the eastern part of the continent south of the ice with temperature departures of no less than 5 6 c along the gulf coast information regarding occurrence of tundra and depression of treeline in the appalachian mountains provides another estimate of summer temperature depression that is in agreement with these results comparison with proxy data from other parts of the world shows that the degree of temperature depression is of about the same degree as that postulated for the tropics rind and peteet 1985 lui and colinvaux 1985 on the basis of the elevational shifts in vegetational zones and snowline quantitative estimates of past temperatures or temperature departures of the type produced here are valuable because they permit comparison with model simulations of the full glacial climate the depression in summer temperatures indicated is lower than most recent modeling results using climap boundary conditions and adds to suggestions that the reconstructed sea surface temperatures used as boundary conditions in simulations of glacial and holocene climates may be in error

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

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

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

recent and ancient fluvial deposition systems in the amazonian foreland basin peru still active sub andean foreland deformation is suggested to have syndepositionally modified the fluvial depositional environments in the peruvian amazonian foreland basin throughout neogene quaternary time modern fluvial aggradation continues to proceed on a large scale c 120 000 km 2 in two differing depositional systems firstly various multistoried floodbasin deposits are derived from the meandering and anastomosing rivers within the subsiding intraforeland basins secondly in the northern part of the pastaza maranon basin the largest known holocene alluvial fan like formation c 60 000 km 2 composed of reworked volcaniclastic debris derived from active ecuadorian volcanoes has been identified the widespread poorly known dissected surface alluvium terra firme which covers the main part of the peruvian amazonian foreland basin shows further evidence of long term foreland deformation and terraces indicate both the effects of tectonism and pleistocene climatic oscillations in northern peru the surface alluvium was deposited by a tertiary fluvial system with palaeocurrents to the west and northwest into the andean foreland basin in southern peru the respective surficial alluvium was part of a post miocene fluvial system flowing northeast into the main amazon basin both systems were gradually abandoned when the eastward migrating andean foreland deformation led to the more distinctive partitioning of the intraforeland basins and the modern drainage system was created

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

late quaternary land snails from the north coast of jamaica local extinctions and climatic change abstract analysis of the land snail faunas and chronology 14c and amino acid epimer ratios of six late quaternary deposits from a site on the north coast of jamaica provides a basis for reconstruction of local extinction events and their relation to paleoclimate paleotemperature estimates are derived from epimerization rates at an interior site and indicate an average holocene late last glacial temperature difference of c 4 5 c size variation in land snails indicates relatively dry conditions during the late last glacial and moist conditions in the late holocene followed by drier conditions beginning sometime during the last millennium the rich faunas of the latter half of the last glacial are represented today at the same site by only a relatively few remnant species two major periods of extinction are identified during latest last glacial to early holocene time a number of species became locally extinct probably due to increased temperatures many survive today in the cooler interior of the island extensive extinctions occurred again during the last millennium these apparently resulted largely from habitat disturbance by man a similar pattern of late holocene extinctions has also been documented for vertebrates on other tropical islands and is also thought to result from human disturbance

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

simulation of the ice age atmosphere january and july means a simulation of the atmospheric state under ice age conditions 18 000 years before present is presented the t21 atmospheric general circulation model agcm originally developed at the european centre for medium range weather forecasts was used for the calculation of six annual cycles results of the near surface climatology 2 m temperature 10 m wind and precipitation averaged over the last five model years are presented the presentation is restricted to january and july means in order to demonstrate the glacial changes in summer and winter the model s response to ice age boundary conditions was quite consistent with paleogeological data on land and with agcm experiments of other studies although the differences between the mean climate states of the model atmosphere under glacial and modern boundary conditions were statistically significant the basic structure of the simulated atmospheric circulation was not altered substantially

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

quantitative estimates of full glacial temperatures in equatorial africa from palynological data this paper presents a new reconstruction of the mean annual temperature obtained from a high altitude pollen sequence in equatorial africa 3 28 s 29 34 e 2240 m it has been achieved with an extended modern african reference data set by adding spectra from 228 new sites and using another selection for pollen taxa the purpose of this paper is to test how the obtained temperature value depends upon the availability of modern analogues the results are in good agreement with those previously published reinforcing the validity of the method the mean standard error is reduced by 0 3 c the mean temperature for the holocene appears 1 4 c warmer than the present and the last glacial maximum 25 18 kyrs bp cooling is better specified at 3 1 9 c a conservative value more consistent with reconstructed sea surface temperature in the equatorial ocean

a review of lateglacial pollen records from ecuador and peru with reference to the younger dryas event abstract published biostratigraphic records from ecuador spanning the lateglacial period of approximately 13 000 to 10 000 bp lack radiocarbon dates delineating the younger dryas and records with extrapolated dates do not exhibit evidence of a climatic reversal for peru fossil pollen data from four sites suggest one and possibly two climatic oscillations in the lateglacial but only one site laguna junin has dates that delimit a short term climatic change and this occurs between 12 000 and 11 000 bp pollen records from laguna baja and rio blanco pond give the strongest indication of a climatic reversal but bracketing dates are not available other pollen records from peru have tenuous dating or data and do not support a lateglacial climatic reversal at the time of the younger dryas

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

the archean lode gold deposits at wiluna western australia high level brittle style mineralization in a strike slip regime the wiluna lode hosted gold deposits comprise a group of individual archean lode hosted gold deposits located in the northern part of the norseman wiluna belt western australia collectively they are the fifth largest gold producer in the yilgarn block the deposits are hosted in a series of very low grade metamorphosed prehnite pumpellyite facies mafic and ultramafic lava flows with abundant primary igneous textures preserved lode hosted gold mineralization is controlled by the wiluna strike slip fault system which consists of north and northeast trending and west and east dipping sets of principal displacement faults and associated subsidiary structures the movement on all faults was dextral with minor oblique reverse or oblique normal motion and maximum apparent displacement of 1 350 m the lode deposits are centered on structural inhomogeneities in the principal displacement faults e g divergent bends and dilational jogs including horsetail splays and or subsidiary structures e g overstepping and intersecting faults the plunge of the ore shoots is generally moderately to steeply to the south and is controlled by the intersections between 1 lithologic layering and fault planes 2 two or more fault planes 3 a fault plane and an earlier formed barren or weakly mineralized quartz reef that is semiconcordant to the regional stratigraphy and 4 a fault plane and a felsic intrusion the shape and orientation of ore shoots becomes more complex downdip where they are controlled by additional features such as drag folds and link structures fault hosted ore breccias including implosion and net veined breccias and brecciated veins are significant loci of gold mineralization in all lode deposits they contain mainly quartz chalcedony carbonates fuchsite and arsenopyrite pyrite and or stibnite mineralization occurs also in shear veins and extensional veins and finely disseminated along foliation planes of the mineral fabric formed oblique to the principal displacement faults textures of implosion breccias at wiluna indicate a history of incremental extensional opening across dilational sites and multiple episodes of brecciation and hydrothermal recementation quartz textures such as vugs cavities comb textures and cockade textured quartz are ubiquitous indicating the preservation of open space due to low effective confining pressure geologic relations indicate that the mineralized strike slip fault system was generated late in the tectonic history of the greenstone belt and that high grade ore zones formed late in the strike slip deformation event the very low grade metamorphic facies and the brittle style of deformation in the host rocks combined with the style of mineralization indicate that the fault and mineralizing system formed in a shallow near surface crustal environment textural evidence combined with the tectonic setting of the lode deposits favors the suction pump mechanism as a fluid infiltration mechanism for the lode hosted gold deposits within the wiluna strike slip fault system