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Tropical forest changes during the late quaternary in African and South American lowlands
Abstract
Arboreal pollen and montane elements of Late Quaternary pollen assemblages from three lacustrine cores (West Cameroon, southeastern Amazonia and central Brazil) are correlated, by the radiocarbon chronology, with other palaeoenvironmental records in Africa and South America.We observe in both continents a well-developed dense forest at 30,000 and 9000 yr B.P. The succession of vegetation types during the Late Quaternary appeared strongly related to the regional conditions: (1) the dense forest was more or less degraded depending on the regions during the last full glacial period (20,000–15,000 yr B.P.); (2) a slow increase of tree elements is evidenced in some areas during the Late Glacial (15,000–10,000 yr B.P.), whereas short-term fluctuations occurred in central Brazil during the same time; (3) a strong regression of the forest during the middle Holocene (6000–5000 yr B.P.), in the southern tropical zone of South America, was in opposition to a full forest development in Africa.In both continents two main features characterize the tropical forest evolution: (1) Montane elements developed in the lowlands during the last glacial period and in some southern or northern regions during the early Holocene; and (2) the climate seasonality was enhanced in several regions since 8500–7500 yr B.P.For a tentative explanation, we relate the cold or cool climate, inferred by palaeoecological evidences in the glacial period and glacial-interglacial transition, to polar air-masses reaching more frequently the tropical zone. This interpretation explains the apparent contradiction between the markedly low temperature of the continental lowlands opposed: (1) at 18,000 yr B.P., to the 1–2°C lower Sea Surface Temperature of tropical oceans and (2) to the global warming during the late glacial. During the middle and Late Holocene, climate evolution was mainly influenced by the latitudinal shift of the ITCZ positions in July and January and, in South America, by short-term changes of the zonal atmospheric circulation.
... The magnitude and location of this seasonal migration has been linked to droughts in the African Sahel, India, Zimbabwe, South Africa, Central America and the Chaco region of South America (Lauer, 1989). Changes in the location and intensity of the ITCZ have also been implicated in late Quaternary climate change in Africa and South America (Servant et al., 1993). These changes include the mid-Holocene dry period that appears to have affected many humid tropical forests approximately 5000 years BP, plus changes in the Gupta, 1988Gupta, 2025 405 Tacajo, Cuba 3-8 Oct 1963 Gupta, 1988Gupta, 1825 1825 La Réunion 7-8 Jan 1966 Landsea, 1999Landsea, 1524 762 Silver Hill, Jamaica 5-7 Oct 1963 Gupta, 1988Gupta, 1248 1248 Pai Shih, Taiwan 10-11 Sept 1963 Gupta, 1988Gupta, 1168 1168 Baguio, Philippines 14-15 July 1911 Gupta, 19881400-800 266-466 Nicaragua 29-31 Oct 1998Ferraro et al., 1999 Monsoon 3388 484 Cherrapunji, India 9-16 June 1876 Gupta, 1988 3213 536 Cherrapunji, India 24-30 June 1932 Gupta, 1988Gupta, 1036 1036 Cherrapunji, India 14 June 1876 Gupta, 1988 493-468 493-468 Various sites in India Gupta, 1988 Frontal systems and inter-tropical convergence zone 2789 930 Bowden Peninsula, Jamaica 22-25 Jan 1960 Gupta, 1988Gupta, 1109 1109 Bowden Peninsula, Jamaica 23 Jan 1960 Gupta, 1988 977 977 Bowden Peninsula, Jamaica 22 Jan 1960 Gupta, 1988 911 304 Maiquetia, Venezuela 14-16 Dec 1999 Pan American Health Organization, 1999 867 867 Cienfuegos, Cuba 1 June 1996 Planos, 1999 magnitude and frequency of ENSO events (see Mahé et al., this volume). ...
... Individual fronts can be several hundred kilometres in length and 10 to 100 kilometres in width and are important in the interchange of temperature and humidity between tropical and mid-latitude regions, especially in the sub-tropics and parts of tropical North America, the South Pacific, the South Atlantic, South Africa and Australia. Cool tropical climates during the late Quaternary glacial period and glacialinterglacial transition have also been linked to an increase in the frequency of polar air masses reaching the tropics (Servant et al., 1993). ...
... Prolonged droughts and fires are an important, but often underestimated, disturbance in humid tropical forests (Brunig, 1969;Wright, 1992;Walsh and Newbery, 1999;Malmer et al., this volume). Paleoclimatic evidence from Africa, Asia, Amazonia, Central America and the Caribbean indicate that most humid tropical forests have experienced numerous fires and extended droughts during the past 10 000 years (Sanford et al., 1985;Hodel et al., 1991;Servant et al., 1993;Guilderson et al., 1994;Mahé et al., this volume). Currently, as much as 200 000 km 2 of the Brazilian Amazon may burn each year and accidental fires may have affected nearly 50% of the remaining un-cleared Amazonian forests . ...
... No caso do fogo, é preciso adotar uma perspectiva histórica para dirimir essa questão. A Tabela 1 apresenta um modelo simplificado da provável evolução dos regimes de queima na região dos cerrados no final do período quaternário, desde o auge do último período glacial, mostrando em linhas gerais que houve muitas mudanças (ver Ab'Saber, 1977;Absy et al., 1993;Servant et al., 1993;Ledru, 1993). O final da última glaciação, antes da chegada do homem nas Américas, caracterizou-se como um período frio e seco, onde a única causa de incêndios eram os raios, o que, aliado à baixa disponibilidade de combustível devido ao clima mais seco e à presença de grandes herbívoros, faz prever uma baixa freqüência de ocorrência de incêndios, concentrados na estação chuvosa (quando os raios ocorrem). ...
... No caso do fogo, é preciso adotar uma perspectiva histórica para dirimir essa questão. A Tabela 1 apresenta um modelo simplificado da provável evolução dos regimes de queima na região dos cerrados no final do período quaternário, desde o auge do último período glacial, mostrando em linhas gerais que houve muitas mudanças (ver Ab'Saber, 1977;Absy et al., 1993;Servant et al., 1993;Ledru, 1993). O final da última glaciação, antes da chegada do homem nas Américas, caracterizou-se como um período frio e seco, onde a única causa de incêndios eram os raios, o que, aliado à baixa disponibilidade de combustível devido ao clima mais seco e à presença de grandes herbívoros, faz prever uma baixa freqüência de ocorrência de incêndios, concentrados na estação chuvosa (quando os raios ocorrem). ...
... A situação mudou com a chegada do Holoceno: o clima tornou-se inicialmente úmido, mas continuou frio, a paisagem passou a ser dominada por florestas de Araucaria e campos (Servant et al., 1993;Ledru, 1993) e os caçadores de megafauna foram substituídos por caçadores-coletores das tradições Itaparica e, posteriormente, Serranópolis, característicos da região do cerrado, que utilizavam armas líticas para a caça da fauna de menor porte, típica do Holoceno (Barbosa e Schmitz, 1998;Guidon, 1992;Schmitz, 1987Schmitz, , 1989Schmitz, , 1990aSchmitz, e 1990bBarbosa e Nascimento, 1990). Os caçadores da tradição Itaparica viviam em pequenos grupos (< 50 indivíduos) com alta mobilidade e baixa densidade (< 0.01 hab/ km2) (Schmitz, 1989). ...
... anos A.P. houve uma expansão da caatinga devido a uma fase climática menos úmida / seca (Figura 4). Essa expansão da caatinga durante o Holocênico médio (~6 Ka A.P.) se deu no período em que a posição verão boreal da ZCIT foi mais ao norte do que hoje, decorrendo em uma fase seca que ocorreu na América do Sul como um todo(SERVANT et al., 1993).Reforçando esse quadro, entre 7.600 e 4.500 anos A.P. parece ter ocorrido o período mais quente do atual período interglacial, fase conhecida como o "ótimo climático", quando a Terra estava, em média, 2 a 3ºC mais quente do que nos dias atuais (SANT'ANNANETO;NERY, 2005). ...
... Depois desse período de expansão da caatinga, entre ~3.200-2.000 anos A.P.(Holoceno Tardio) até o presente, a tendência geral da evolução de clima é caracterizada por um deslocamento da ZCIT para o sul da América do Sul , onde registros de isótopos de carbono sugerem uma nova expansão florestal e uma fase mais úmida, com os eventos regionais secos apresentando-se menos frequentes, mas que continuariam afetando ocasionalmente a dinâmica vegetacional(PESSENDA et al., 2010;SERVANT et al.), associado à grande extensão territorial (1.540.827 km 2 ) e à diversidade geográfica, configuraram o Nordeste do Brasil como uma região climatologicamente complexa, o que não se traduz em grandes diferenciações térmicas. ...
Belonging to the humid mountain range of the northeastern semi-arid region, Ibiapaba presents gaps in its natural history and, therefore, there is a need for greater knowledge about its natural characteristics and singularities, for example. An important step to fill this gap is the integrated knowledge between the past and present climatic patterns and their hydrographic reflections in the hydrographic basins, a fundamental context to analyze the relationship between the geological substrates and the reliefs that make up the Ibiapaba landscape, in the Region of the state of Ceará. Therefore, the main objective of this work is to perform a hydro-climatic characterization of Ibiapaba and its adjacent areas. Methodologically, the theoretical assumptions of related topics were used, with geomorphological science as guiding element. Technically, the methodological contingent relied on the cabinet stages, with detailed and careful bibliographical and cartographic surveys, in the field surveys and in the integrated analysis of all the data in the laboratory stage, with emphasis on the thematic mappings of Ibiapaba and region. The results obtained, presented in the hydro-climatic characterizations, in the theoretical and empirical discussions made, and in the elaborated graphic materials, allow us to start an analysis of the geomorphological picture of Ibiapaba, in the northwest region of the state of Ceará.
... According to the first hypothesis, the Wisconsinan stage was characterized by a regression of the forests and an expansion of the tropical and subtropical open formations in South America. For some authors, this Neotropical forest regression was the result of the driest Wisconsinan periods (van der Hammen, 1974(van der Hammen, , 1982Ab'Saber, 1982;Ledru, 1993;Servant et al., 1993;van der Hammen & Absy, 1994). For others, and especially palynologists, grassland expansion was a response to both cold and dry phases during the late Quaternary, especially in the south and south-east Brazilian landscape (Markgraf & Bradbury, 1982;Markgraf, 1991;Soubies et al., 1991;Behling, 1993Behling, , 1995Behling, , 1997aBehling, ,b, 2002. ...
... According to the first hypothesis, the Wisconsinan stage was characterized by a regression of the forests and an expansion of the tropical and subtropical open formations in South America. For some authors, this Neotropical forest regression was the result of the driest Wisconsinan periods (van der Hammen, 1974(van der Hammen, , 1982Ab'Saber, 1982;Ledru, 1993;Servant et al., 1993;van der Hammen & Absy, 1994). For others, and especially palynologists, grassland expansion was a response to both cold and dry phases during the late Quaternary, especially in the south and south-east Brazilian landscape (Markgraf & Bradbury, 1982;Markgraf, 1991;Soubies et al., 1991;Behling, 1993Behling, , 1995Behling, , 1997aBehling, ,b, 2002. ...
... The persistence of arid conditions across North Africa during deglaciation is recorded in deep sea core and terrestrial sediments that attest to lowland rainforest and savanna degradation (e.g. Brun, 1989;Fontugne et al., 1993;Servant et al., 1993), reduced flow of the Nile River (Rossignol-Strick, 1983;Fontugne et al., 1993) and lake levels across eastern Africa (Street and Grove, 1979). Dry conditions across intertropical Africa through 15 ka BP resulted from diminished regional rainfall regimes, which have been attributed to the weakened Afro-Asian monsoon system (Kutzbach and Street-Perrott, 1985;Prell and Kutzbach, 1987). ...
... Existing data are too sparse and disputable to produce convergent views on late Quaternary palaeoecology and palaeohydrology in the Amazon basin (Kadomura 1995). Palaeoecological and sedimentological evidence from the rainforest portion of the basin suggests dry phases during 6000-3000 years before present (BP) and 2700-400 years BP in the Brazilian Amazon (Sanford et al. 1985), and 4300-4200 and 3800-3150 years BP in the Ecuadorian Amazon (Liu and Colinvaux 1988), probably related to the ENSO circulation (Servant et al. 1993). Freshwater influx to the Amazon cone during the period 13 600-6000 years BP peaked c. 9600 years BP, probably in association with deglacial pluvial activity (Showers and Bevis 1988). ...
The climate and hydrology of each of the world's major river drainages are inherently diverse because these drainages cover such large areas that they encompass diverse atmospheric circulation patterns, geology, topography, vegetation, and land use. The peak unit discharge of major rivers also reflects precipitation‐generating mechanisms. The flood hydroclimatology of the major rivers of low latitudes, such as the Amazon, Congo, and Zambezi, shows low variability in monthly and annual peak discharge. Rivers of arid and semiarid regions are distinguished by generally low values of peak discharge per unit drainage area because some part of the basin does not effectively contribute surface runoff during peak discharges. The rivers of the mid‐latitudes of North America and Europe have climates in which diverse circulation patterns produce precipitation. The rivers of East and Southeast Asia, which drain mostly the mid‐latitudes, have seasonally or perennially barotropic climates.
... Par exemple, les données au sein du bassin du Congo sont quasiment inexistantes et l'évolution précise du climat pour cette région est très mal documentée et les données souffrent d'une faible résolution temporelle et de grandes incertitudes de datation à l'échelle du continent (Gasse, 2000). Les modèles de l'évolution de la température pendant les périodes glaciaires sont donc parfois très différents selon qu'ils sont déduits des températures de surface de l'océan ou de données paléoécologiques (Servant et al., 1993). Ainsi, les variations climatiques décrites dans le Tableau I-2 doivent être considérées de manière globale pour l'Afrique et ne reflètent pas les variations spatiales et temporelles qui ont pu exister à des échelles plus locales (Fjeldså and Lovett, 1997). ...
The origin of the distribution of plant species endemic to western Central Africa (WCA), an area dominated by forests and among the continent’s richest in species, has long been questioned by biogeographers. It has been suggested that the ranges of forest species contracted to within a few forest refugia during periods of glacial maxima, when the climate was cooler and drier, and that some species may not have followed the re-expansion of forest when climatic conditions became more favorable. The concentration of forest species endemic to restricted areas would thus reflect the former areas of forest refugia.
The identification of areas rich in endemic and/or endangered species is also crucial for implementing efficient conservation strategies.
This thesis has two objectives:
(i) to investigate the extent to which the current distribution of forest species endemic to WCA can be explained by their persistence within forest refugia during the Quaternary ice-ages;
(ii) to analyse whether the conservation strategies adopted in Gabon, a country located at the centre of WCA, protect the country’s endemic and endangered plant species.
To achieve these objectives, a compilation and verification of 19,876 occurrences of the 1,145 taxa (species and sub-species) endemic to WCA were performed. In total, about 13% of the flora of WCA is endemic to the region, 88% of which are forest taxa. The results show that the dispersal capacity of taxa and the forest refugia hypothesis seem to be explanatory factors for the differences observed in the rate of endemism between growth forms and between taxonomic groups.
The analysis of the spatial distribution of endemic forest species revealed the presence of ten areas of forest endemism in WCA, six of which are partly congruent with the mountain ranges and the coastal part of the region, areas proposed in the literature as forest refugia. The results suggest, however, that other forest refugia may have existed further inland.
Based on the assumption that forest refugia were maintained during glacial maxima thanks to the presence of accessible moisture present in the soil or the air, we defined four types of hydrological refugia in which the forest cover could have been maintained: (i) flat areas along major rivers, (ii) valley bottoms in hilly lowland landscapes, (iii) coastal areas, and (iv) mountainous areas exposed to oceanic air masses. Their distribution explains the presence of the different areas of forest endemism found in the region but does not fully account for the patterns of richness in endemic species. The hypothesis of hydrological refugia is thus relevant for explaining the current distribution of forest species endemic to WCA, but other factors such as the distribution of environmental gradients or the impact of humans on forests must also be considered.
Finally, the preliminary assessment of the risk of extinction of species according to the Categories and Criteria of the International Union for Conservation of Nature’s Red List, using a semi-automated method developed for this work, and an analysis of species distributions, showed that the National Parks of Gabon fail to protect a significant portion of the country’s endemic and endangered species, although a lack of botanical exploration in most parks may partially explain this finding.
... Plus récemment, les périodes Pliocène (5 à 1,8Ma) et pléistocène (1,8Ma à 0,01Ma) caractérisées par de nombreuses oscillations climatiques ont connu une succession d'évènements glaciaires et interglaciaires qui ont eu des répercutions à l'échelle mondiale. Ces cycles ont joué un rôle majeur en Amazonie sur le climat (Stute et al., 1995b ;Harris et Mix, 1998 ;Servant et al., 1993;Hammen, 1974;Hammen et Absy, 1994 ;Colinvaux et al., 1996 ;Häggi et al., 2017, Zhang et al., 2017, Fontes et al., 2017 notamment sur les régimes des précipitations (Colinvaux et Liu, 1987, Wesselingh et Hoon, 2011, et le type de végétation (Clapperton, 1993 ;Häggi et al., 2017). Ces évènements glaciaires mais aussi géologiques ont affecté la dynamique fluviale (changements du cours des rivières), en particulier les phases d'érosions / sédimentations (Baker et Fritz, 2015) et la distribution des espèces (Dumont et al., 1990 ;Hubert et Renno, 2006 ;Oberdorff et al., 2019). ...
Nous avons choisi le modèle Apistogramma agassizii pour tester la spéciation rapide en sympatrie chez les cichlidés Amazoniens. A. agassizii partage de nombreuses similitudes avec les cichlidés Haplochromines qui sont des exemples remarquables de radiations adaptatives chez les vertébrés. Tout comme les Haplochromines A. agassizii est philopatrique, et présente : un important polymorphisme de couleur avec un dimorphisme sexuel marqué, un choix de partenaire différentiel, et une garde parentale. Nous avons échantillonné 1170 individus d’A. agassizii provenant de 36 popsites (unité de collecte la plus petite) au Loreto en Amazonie péruvienne. La variabilité génétique d'A. agassizii a été étudiée en utilisant 2 marqueurs mitochondriaux (COI et Cyt b) et 10 locus microsatellites. Les 1170 individus ont été génotypés parmi lesquels 104 ont été séquencés pour les deux marqueurs mitochondriaux. Au total 44 haplotypes ont été obtenus à partir des séquences concaténées. Les analyses ont mis en évidence 3 « species flocks » (nommés Sp1, Sp2 et Sp3) vicariants, qui auraient commencé à diverger de leur ancêtre commun le plus récent (MRCA) il y a 1,83 Ma (calibration à partir de fossiles). Les degrés de différentiation génétique des 3 « species flocks » s’organiseraient de façon fractale suivant la hiérarchisation du réseau hydrographique. Les valeurs de différenciations génétiques (estimateur θ du FST) que nous avons observé dans les « species flocks », n’avaient encore jamais été observés sur de si petites échelles géographiques pour un poisson en Amazonie (Sp1 : 0,04 - 0,37, moy. = 0,16 ± 0,06 (σ) ; Sp2 : 0,08 - 0,40, moy. = 0,17 ± 0,09 (σ)). Les rivières Marañon, l’Ucayali et l’Amazone, constitueraient le niveau hiérarchique de rang 1 en limitant la dispersion des 3 « species flocks », favorisant la spéciation allopatrique. Les microbassins hydrographiques constitueraient le niveau de hiérarchisation inférieur de rang 2, avec des différenciations génétiques d’amplitude variable structurant les species flocks en sympatrie. Le terme de « sympatrie » étant ici employé au sens large. Enfin, à l’intérieur des microbassins, les ruisseaux constitueraient le niveau hiérarchique de rang 3 dans lequel des proto-espèces (unité génétique dont le processus de spéciation peut être réversible) divergeraient en sympatrie stricte. L’histoire évolutive du « species flock » Sp1 a été conditionnée par des événements hydro-géomorphologiques multiples qui auraient alternativement favorisés des évènements de fragmentation ou de dispersion induisant des mélanges de populations mises en contacts secondaires. Des expérimentations sur le mode du choix de partenaire ont été réalisées en tenant compte des 3 niveaux hiérarchiques identifiés. Au niveau de hiérarchie de rang 1 (sous-bassin), les femelles Sp1 et Sp2 ont choisi préférentiellement des mâles de leur propre « species flock » (Sp1 : p-value = 0,0005 ; Sp2 : p-value = 0,0029). Pour le rang 2 nous avons testé le choix des femelles Sp1 en leur proposant un mâle de leur ruisseau (même microbassin) et un mâle d’un ruisseau d’un autre microbassin. Pour le rang 3 nous avons testé le choix des femelles Sp1 en leur proposant un mâle de leur ruisseau et un mâle d’un autre ruisseau appartenant au même microbassin. Dans les deux configurations, les femelles marquaient généralement une préférence sexuelle selon l’origine des mâles, sans que celle-ci ait pu être testée de façon suffisamment approfondie pour corroborer que la sélection sexuelle jouerait un rôle moteur dans le processus de spéciation en sympatrie. Les mécanismes évolutifs mis en évidence dans le modèle Apistogramma agassizii pourraient également être impliqués dans la diversification d’autres espèces de Cichlidés, voire d’autres groupes d’organismes en Amazonie et expliquer pour partie l’hyper diversité spécifique dans le plus grand bassin hydrographique du monde.
... During the Pleistocene there were about 21 major fluctuations in temperature, precipitation, atmospheric composition and hydrology, this dynamic environment being reflected by changes in species distributions. The most recent of these fluctuations, characterised by a maximum drying and cooling of the climate, occurred about 21 000 radiocarbon years before present ( 14 C yr BP) and ended about 10 000 14 C yr BP (Hamilton 1982;Bonnefille et al. 1990;Servant et al. 1993). Species responded to climatic change by exhibiting three responses: acclimatisation, adaptation or migration, with a species having to respond in one way, or a combination of ways, to persist. ...
Phylogeny is a potentially powerful tool for conserving biodiversity. This book explores how it can be used to tackle questions of great practical importance and urgency for conservation. Using case studies from many different taxa and regions of the world, the volume evaluates how useful phylogeny is in understanding the processes that have generated today's diversity and the processes that now threaten it. The novelty of many of the applications, the increasing ease with which phylogenies can be generated, the urgency with which conservation decisions have to be made and the need to make decisions that are as good as possible together make this volume a timely and important synthesis which will be of great value to researchers, practitioners and policy-makers alike.
... The start of the Holocene around 10000 BP coincided almost exactly with the last phase of the maximum expansion of the rain forests in all the equatorial zones (Servant et al., 1993). In South America, the history of the eastern part ofthe Amazon forest recorded at Carajas shows that after a first expansion between 10000 and 8000 BP, the forest diminished considerably until about 4000 BP with the driest period occurring between 6000 and 5000 BP (Siffedine et al., 1994). ...
... Même si pendant le DMG, les glaciers ne sont pas descendus jusqu'aux zones tropicales, ces régions ont cependant connu des modifications climatiques. En Afrique centrale, le climat plus sec et plus froid, confirmé par la présence à faible altitude d'une végétation montagnarde entre 30 000 et 10 000 ans BP (Maley, 1991;Servant et al., 1993), aurait entrainé des contractions et une fragmentation de la forêt humide dans des zones refuges situées en Guinée équatoriale, au Cameroun, au Gabon, au ...
La distribution actuelle des espèces d’arbres des forêts tropicales humides amazoniennes est le résultat de processus dynamiques sur lesquels les perturbations passées, d’origine climatique ou anthropique, peuvent avoir jouées un rôle majeur. Des outils génétiques actuels associés à des méthodes d’analyses puissantes permettent d’inférer et de dater l’empreinte génomique des évènements démographiques subits par les populations naturelles. Le but de cette thèse est de contribuer à une vision d’ensemble de l’évolution démographique de la forêt tropicale humide du plateau des Guyanes pendant le Quaternaire. J’ai d’abord étudié les propriétés des microsatellites (répétitions simple de séquences, SSR) et les limites de leurs applications en tant que marqueurs moléculaires en phylogéographie. Plusieurs allèles SSR contenu dans diverses populations de trois genres d’arbres différents ont été séquencés et le polymorphisme, porté par chaque partie des fragments ADN contenant des SSR, a été étudié. J’ai observé que la quantité de variation génétique était aussi importante dans les régions encadrant le microsatellite que dans le SSR lui-même. Ces régions ont contribué significativement à l’information phylogénétique et ont parfois été la principale source de différenciation entre les individus et les populations. Cette expérience m’a permis de choisir la meilleure stratégie à adopter pour traiter les données de variation génétique des SSR dans des modèles démographiques. J’ai ensuite utilisé ces marqueurs SSR et des méthodes ABC pour étudier l’impact local des occupations humaines précolombiennes sur les forêts en Guyane française. Les changements démographiques subis par des populations de quatre espèces d’arbres ont été comparés entre cinq sites occupés et quatre sites non perturbés. Les perturbations naturelles et induites par l’homme ont contribué à la mise en place de la diversité observée actuellement, comme le suggère des signatures d’effet fondateur détectées sur les sites perturbés ou non. Néanmoins, la taille des populations fondatrices semble être proportionnelle à l’impact humain estimé, indiquant que les perturbations anthropique ont induit un remplacement des populations plus important que les processus écologiques naturels. Pour terminer, j’ai mené une étude de démographie comparative d’espèces d’arbres tropicaux à l’échelle régionale de la Guyane française. Cinq scénarios démographiques ont été testés pour neuf populations de huit espèces en utilisant des microsatellites nucléaires et des séquences chloroplastiques. Les changements démographiques les plus récents détectés convergent vers une signature générale d’expansion de la forêt humide depuis probablement la dernière glaciation. Cela peut suggérer, soit que la composition des forêts du plateau des Guyanes était différente, ou que le couvert forestier était moins étendu. Ce résultat constitue le premier indice de changement passé, probablement d’origine climatique, subi par les communautés forestières du plateau des Guyanes ; qui n’avait jusque-là pas pu être testé avec des méthodes standards en raison d’un manque de fossiles. En conclusion, j’ai recueilli des preuves génétiques que les perturbations, à différents échelles de temps, ont marqué la structure et la composition génétique des forêts du plateau des Guyanes. L’application de ces méthodes à d’autres régions Néotropicales pourrait contribuer, à l’avenir, à reconstruire l’histoire écologique de la forêt amazonienne de façon détaillée.
... This species-rich family initially diversified in tropical rainforest biomes, but the Thamnophilini tribe shifted to more open savannah habitats along the periphery of the Amazon Basin, beginning in southern Brazil (Bravo 2012;Belmonte-Lopes 2013). This would probably have required some flexibility in its thermal tolerance, as inhabitants of this region would have had to endure periods of cold weather caused by southern polar winter winds, notably in the Pleistocene epoch (Servant et al. 1993). There is a good deal of empirical evidence to suggest that species differ greatly in their ability to tolerate cold: most species prefer hotter temperatures but differ greatly in their abilities to tolerate cold, which may be a specialized condition that allows some groups to take advantage of uncontested food resources that exist (at least for part of the year) at high latitudes (Araújo et al. 2013). ...
Several studies have shown that climatic change has been accelerating due to human activities, leading to dramatic effects on biodiversity. Modeling studies describe how species have reacted in the past to climatic change, and this information can help us to understand the degree of biotic susceptibility to current and future climatic change. This work aims to determine the effects of past, current and future climatic changes on the geographic distribution of the species complex Thamnophilus punctatus, a bird clade widely distributed across Neotropical dry forests. We also investigate if species that are phylogenetically similar have comparable climatic niches and, consequently, can be expected to respond similarly to climatic change. For this purpose, we calculated similarity, niche overlap, equivalence and genetic distance between all species, modeling their geographic distributions during the Last Glacial Maximum (LGM) as well as under current conditions and future (2050–2080) scenarios. Our results indicate that there are differences in responses to climatic changes from the LGM to the present among the five species of the T. punctatus complex and that the niches in the measured dimensions are not conserved among the studied species. We therefore suggest that the adequate environmental space of taxa of a widely distributed lineage can be shaped in distinct way, regardless of how closely related their species are or how much their niches overlap. Competitive exclusion in zones of contact is an important factor determining the geographical range of the species of the Thamnophilus punctatus complex, particularly for the very closely related species T. sticturus, T. pelzelni and T. ambiguus.
... It means that whatever the presence of other inselbergs and the cover of open-canopy forest in the vicinity of a given inselberg (for instance within the so-called archipelagos), only its size and size-related factors (habitat diversity and sampling effort), as well as environmental filters (rainfall and altitude), contribute to increase or decrease the total number of plant species it harbours. While several studies showed that during drier periods of the Holocene savanna-like vegetation covered large areas of the Guianan Shield (Servant et al. 1993, Freycon et al. 2010, Rull and Montoya 2017, present-day rock-savanna vegetation should be considered as a relict constellation of isolated fragments of xeric vegetation embedded in a more or less continuous rain-forest cover (de Granville 1982, Prance 1996. Even inselbergs distributed in archipelagos do not seem to exchange diaspores to a significant extent (Barbará et al. 2007, Pinheiro et al. 2014. ...
Disentangling the multiple factors controlling species diversity is a major challenge in ecology. Island biogeography and environmental filtering are two influential theories emphasizing respectively island size and isolation, and the abiotic environment, as key drivers of species richness. However, few attempts have been made to quantify their relative importance and investigate their mechanistic basis. Here, we applied structural equation modelling, a powerful method allowing test of complex hypotheses involving multiple and indirect effects, on an island-like system of 22 French Guianan neotropical inselbergs covered with rock-savanna. We separated the effects of size (rock-savanna area), isolation (density of surrounding inselbergs), environmental filtering (rainfall, altitude) and dispersal filtering (forest-matrix openness) on the species richness of all plants and of various ecological groups (terrestrial versus epiphytic, small-scale versus large-scale dispersal species). We showed that the species richness of all plants and terrestrial species was mainly explained by the size of rock-savanna vegetation patches, with increasing richness associated with higher rock-savanna area, while inselberg isolation and forest-matrix openness had no measurable effect. This size effect was mediated by an increase in terrestrial-habitat diversity, even after accounting for increased sampling effort. The richness of epiphytic species was mainly explained by environmental filtering, with a positive effect of rainfall and altitude, but also by a positive size effect mediated by enhanced woody-plant species richness. Inselberg size and environmental filtering both explained the richness of small-scale and large-scale dispersal species, but these ecological groups responded in opposite directions to altitude and rainfall, that is positively for large-scale and negatively for small-scale dispersal species. Our study revealed both habitat diversity associated with island size and environmental filtering as major drivers of neotropical inselberg plant diversity and showed the importance of plant species growth form and dispersal ability to explain the relative importance of each driver.
... The start of the Holocene around 10000 BP coincided almost exactly with the last phase of the maximum expansion of the rain forests in all the equatorial zones (Servant et al., 1993). In South America, the history of the eastern part ofthe Amazon forest recorded at Carajas shows that after a first expansion between 10000 and 8000 BP, the forest diminished considerably until about 4000 BP with the driest period occurring between 6000 and 5000 BP (Siffedine et al., 1994). ...
... All studies dealing with the late glaciation and Holocene climates in southern Argentina record two major cold reversals (between 13.1 and 11.1 ka BP) and a series of neoglaciations until the present (e.g., Mercer, 1968;Iriondo and Garcia, 1993;Andres et al., 2003;Glasser et al., 2004;Borromei et al., 2007;Haberzettl et al., 2007;Aniya, 2013). In summary, the climate was dominantly dry with polar winds reaching Amazonia (Kronberg and Benchimol, 1992;Servant et al., 1993;Carneiro Filho et al., 2002). A mantle of sand was spread over Argentina during the late glaciation and early Holocene (Carignano and Cioccale, 2005), which is confirmed by luminescence dating of dune fields in San Luis Province (≈33°S) Forman, 2007, 2016). ...
Calcareous dust occurs in Argentina as layers and pockets closely associated with Pleistocene deposits and periglacial features from southernmost Patagonia to at least the Mendoza Precordillera and has been traditionally interpreted as a soil horizon resulting from postdepositional pedogenesis during interglacials. Detailed field and microscopic observations and sedimentological and geochemical analyses of more than 100 samples collected from lower to upper Pleistocene deposits between 51°S and 33°S and from near sea level to 2800 m asl allow us to interpret the dust as synchronous with the host sediment. All observations and analyses lead us to conclude that: (1) the cryogenic morphology and the chemical signatures of the calcite component show that the dust is glaciogenic, (2) the dust was carried by southeasterly Antarctic winds, and (3) it was deposited over most of southern and central Argentina. Field observations, geomorphic evidence, and radiocarbon dates suggest that the dust was deposited during several Pleistocene glacial episodes.
... A northern and lowland expansion of taxa adapted to colder climates (Araucaria, Podocarpus), with a consequent retraction of the tropical forest might have occurred (Behlig, 1998;Colinvaux et al., 1996;Klein, 1984;Servant et al., 1993). The existing patches of Araucaria forest in high altitude areas surrounded by tropical moist forest could represent contemporary refuges of the flora of a colder past climate (Veloso & Góes-Filho, 1982). ...
... BiogeoBears did not identify a single geographical area of origin for the fluvicolines, but Borregaard et al. (2014) found a slight shift in the early Tyrannidae radiation from Amazonian forest to subtropical savannas. We may assume that this region experienced periods with occasional spells of cold weather from when accumulation of ice on Antarctica started in the mid-Miocene (see Servant et al. 1993 for the Pleistocene), and this may have selected for increased thermogenic performance. ...
The fluvicoline New World flycatchers (Subfamily Fluvicolinae, family Tyrannidae) inhabit a broad range of forest and non-forest habitats in all parts of the New World. Using a densely sampled phylogeny we depict the diversification and expansion of this group in time and space. We provide evidence that a shift in foraging behaviour allowed the group to rapidly expand in a wide range of tropical and subtropical habitats in South America. The results support that four main clades expanded into and specialized to distinct habitats and climates (closed to open, and warm to cold), respectively, and subsequently underwent vicariant speciation within their respective ecoregions. The group soon reached a significant species diversity over virtually all of South and North America, and with parallel trajectories of speciation slow-down in all four clades. The genus Muscisaxicola is an exception, as it invaded the most inhospitable and barren environments in the Andes where they underwent rapid diversification in the Plio-Pleistocene.
... It should also be noted that this sudden opening up of the rain forest constitutes the beginning of a fragmentation that occurred in an interglacial period, i.e. in a globally warm phase. In the Amazon forest a comparable destruction phase occurred in the eastern part during the middle Holocene, between about 8000 and 4000 years BP, as has been shown by pollen analyses of the Carajas lake sediments and also by the presence of large quantities of charcoal fragments (Absy et al. 1991;Servant et al. 1993;Martin et al. 1993). From a chronological point of View, this regression of the Amazon forest is out of phase with equatorial Africa. ...
... It should also be noted that this sudden opening up of the rain forest constitutes the beginning of a fragmentation that occurred in an interglacial period, i.e. in a globally warm phase. In the Amazon forest a comparable destruction phase occurred in the eastern part during the middle Holocene, between about 8000 and 4000 years BP, as has been shown by pollen analyses of the Carajas lake sediments and also by the presence of large quantities of charcoal fragments (Absy et al. 1991;Servant et al. 1993;Martin et al. 1993). From a chronological point of View, this regression of the Amazon forest is out of phase with equatorial Africa. ...
This paper gives a historical overview of the African rain forest from its origins, towards, the end of the Cretaceous period. The areas around the Gulf of Guinea, in particular from Ivory Coast to Nigeria and especially Cameroon, Gabon and Congo, appear to have been already occupied at this time by wet tropical forest formations mainly composed of Angiosperms. In the course of the Tertiary period the combined effect of the equator being situated further north than now and the development of the Antarctic ice cap favoured the development of wet tropical conditions over a large part of North Africa. Towards the end of the Tertiary, the equator reached its present position and the northern hemisphere ice caps appeared, and these phenomena resulted in the disappearance of the forest formations spread across the north of Africa, and the concentration of these formations near the equatorial zone around the Gulf of Guinea and in the Congo-Zaire basin. From 800 000 years ago onwards the marked glacial variations at middle and high latitudes in both hemispheres, lowered temperatures in equatorial areas and brought arid climates at times of maximum glacial extension. -Author
... Es decir, si bien en estos ecosistemas de bosques húmedos tropicales (tierras altas y bajas) hay indicios de que los grupos humanos auspiciaron, desde muy temprano, nuevas estrategias de producción, incluyendo el desarrollo del cultivo de plantas, estas funcionaron como ajustes adaptativos y no únicamente como revulsivos evolutivos, como algunas veces se ha pretendido cargar el concepto de producción de alimentos. Estos ajustes, probablemente, estuvieron asociados a ecosistemas que sufrieron cambios importantes, como la expansión de los bosques húmedos y el aumento de plantas C 3, entre las que se incluyen la mayoría de los ancestros de los cultivos neotropicales (Piperno 2006(Piperno , 2011Servant et al. 1993). ...
In the 1960s Reichel Dolmatoff applied the evolutionary model of the American archeology to order the prehispanic record in Colombia. Nonetheless, the Archaic period was loosely defined because of the lack of data existing at the time. Fifty years after that things have changed and we now have robust evidences to be able to understand this key period in the prehispanic history, characterized by its cultural diversity and the adaptive strategies plasticity displayed by the different human groups, among which the origin of food production stands out.
... The start of the Holocene around 10000 BP coincided almost exactly with the last phase of the maximum expansion of the rain forests in all the equatorial zones (Servant et al., 1993). In South America, the history of the eastern part ofthe Amazon forest recorded at Carajas shows that after a first expansion between 10000 and 8000 BP, the forest diminished considerably until about 4000 BP with the driest period occurring between 6000 and 5000 BP (Siffedine et al., 1994). ...
There are many uncertainties in the rainforests and climate variability relationships knowledge under tropical latitudes, This work proposes an up-to-date review focused on this subject through three axes : rainforests and tropical climate history, forests and climate relationships at low latitudes including vegetation modelling in GeMs and recent climate variability in forest areas, These three parts point out many interrogations and polemical aspects about an anthropogenic impact on vegetation and climate. A large knowledge about climate and vegetation interactions is indispensable for estimating human implications in climate and land cover changes, mainly through modelling studies.
... Es decir, si bien en estos ecosistemas de bosques húmedos tropicales (tierras altas y bajas) hay indicios de que los grupos humanos auspiciaron, desde muy temprano, nuevas estrategias de producción, incluyendo el desarrollo del cultivo de plantas, estas funcionaron como ajustes adaptativos y no únicamente como revulsivos evolutivos, como algunas veces se ha pretendido cargar el concepto de producción de alimentos. Estos ajustes, probablemente, estuvieron asociados a ecosistemas que sufrieron cambios importantes, como la expansión de los bosques húmedos y el aumento de plantas C 3, entre las que se incluyen la mayoría de los ancestros de los cultivos neotropicales (Piperno 2006(Piperno , 2011Servant et al. 1993). ...
In the 1960s Reichel Dolmatoff applied the evolutionary model of the American archeolo- gy to order the prehispanic record in Colombia. Nonetheless, the Archaic period was loosely defined because of the lack of data existing at the time. Fifty years after that things have changed and we now have robust evidences to be able to understand this key period in the prehispanic history, characterized by its cultu- ral diversity and the adaptive strategies plasti- city displayed by the different human groups, among which the origin of food production stands out.
... The start of the Holocene around 10000 BP coincided almost exactly with the last phase of the maximum expansion of the rain forests in all the equatorial zones (Servant et al., 1993). In South America, the history of the eastern part ofthe Amazon forest recorded at Carajas shows that after a first expansion between 10000 and 8000 BP, the forest diminished considerably until about 4000 BP with the driest period occurring between 6000 and 5000 BP (Siffedine et al., 1994). ...
There are many uncertainties in the rainforests and climate variability relationships knowledge under tropical latitudes, This work proposes an up-to-date review focused on this subject through three axes : rainforests and tropical climate history, forests and climate relationships at low latitudes including vegetation modelling in GeMs and recent climate variability in forest areas, These three parts point out many interrogations and polemical aspects about an anthropogenic impact on vegetation and climate. A large knowledge about climate and vegetation interactions is indispensable for estimating human implications in climate and land cover changes, mainly through modelling studies.
... O Holoceno médio é reconhecido como um período de alterações globais particularmente significativas (STEIG, 1999). No Brasil os efeitos dessas alterações foram registrados em várias regiões: expansão gradual da vegetação florestada após 7000 anos AP na área de Cerrado (LEDRU et al., 2001); condições mais secas para a região norte por volta de 4.300 a 3.150 anos A.P. e também durante o intervalo de 1.500 a 800 anos A.P. (SERVANT et al., 1993); expansão da Floresta de Araucária, a partir de 3.000 anos A.P. e o paulatino avanço das espécies tropicais sobre os campos que outrora predominavam as área do Planalto Meridional (BEHLING, 1995;1997;2002;BAUERMANN;BEHLING, 2009;BERTOLDO;De OLIVEIRA, 2014). Estudos indicam que o clima foi o pilar para as migrações e desenvolvimento das florestas no Sul do Brasil, o tipo de solo, ação do fogo e o pastejo ainda atuam promovendo variações na paisagem e desenhando o limite Floresta/Campo (JARENKOW; BUDKE, 2009). ...
... O Holoceno médio é reconhecido como um período de alterações globais particularmente significativas (STEIG, 1999). No Brasil os efeitos dessas alterações foram registrados em várias regiões: expansão gradual da vegetação florestada após 7000 anos AP na área de Cerrado (LEDRU et al., 2001); condições mais secas para a região norte por volta de 4.300 a 3.150 anos A.P. e também durante o intervalo de 1.500 a 800 anos A.P. (SERVANT et al., 1993); expansão da Floresta de Araucária, a partir de 3.000 anos A.P. e o paulatino avanço das espécies tropicais sobre os campos que outrora predominavam as área do Planalto Meridional (BEHLING, 1995;1997;2002;BAUERMANN;BEHLING, 2009;BERTOLDO;De OLIVEIRA, 2014). Estudos indicam que o clima foi o pilar para as migrações e desenvolvimento das florestas no Sul do Brasil, o tipo de solo, ação do fogo e o pastejo ainda atuam promovendo variações na paisagem e desenhando o limite Floresta/Campo (JARENKOW; BUDKE, 2009). ...
No período Quaternário, o Sul do Brasil passou por instabilidades paleoclimáticas que geraram mudanças nas paisagens. Estudos pedológicos, sedimentológicos e de reconstrução paleoambiental têm sido desenvolvidos pelo grupo Gênese e Evolução de Superfícies Geomórficas e Formações Superficiais (UNIOESTE-FB) para investigar a hipótese da influência paleoclimática na configuração da paisagem do Sudoeste do Paraná e Noroeste de Santa Catarina. Visando contribuir com estes estudos, objetiva-se especificamente investigar possíveis mudanças na comunidade de plantas que compunha a paleovegetação da região. Para isso são empregadas análises isotópicas (δ13C da matéria orgânica do solo e datação 14C da fração humina) e pedológicas. As análises físicas e químicas de solo permitiram caracterizar e classificar o solo estudado como um Cambissolo húmico. Os valores de δ13C da MOS variam de -16,23‰ a -23,61‰. Assim é possível inferir que este solo registra uma fase em que a vegetação era formada por uma comunidade predominantemente de plantas C4 (vegetação mais aberta que atual), possivelmente no Holoceno Médio (6.235-6.215 anos Cal. AP), evoluindo para uma vegetação mista, mais florestada e semelhante a atual, composta predominantemente por plantas C3 no Holoceno Tardio (de 840 e 835 anos cal. A.P) que evoluiu para a atual Floresta Ombrófila Mista.
... With the exception of the Cerrado site, the Rondonia data show the same pattern. These results are in agreement with other studies that document the occurrence of a fully developed tropical forest in the southern zone of South America, between 10,000 and 9500 yr BP and 8000 yr BP (Absy et al. 1991;Van der Hammen 1991;Servant et al. 1993). ...
This study, which was carried out in the southern Brazilian Amazon region (Rondônia state and Humaitá, Southern Amazon state), presents and discusses the significance of carbon isotope data measured in soil profiles collected across natural boundaries of forest to savanna vegetation. The main objective of this study was to evaluate the expansion-regression dynamics of these vegetation units in relation to climate changes during the Holocene. 14C data from charcoal, soil organic matter (SOM) and its component humin fraction indicate that the organic matter in the studied soils is essentially Holocene in origin. 13C data indicate that C3 type plants were the dominant vegetation at all study areas in the early Holocene, and during the entire Holocene, in the forest sites of Central Rondônia state and in the forest site 50 km from the city of Humaitá. 13C data also indicate that C4 plants have influenced significantly the vegetation at the transitional forest and the Cerrado (wooded savanna) sites of Southern Rondônia state and the forest ecosystem located 20 km from the Humaitá city. These typical C4 type isotopic signatures probably reflect a drier climate during the mid-Holocene. The 13C records representing probably the last 3000 yr show an expansion of the forest, due to a climatic improvement, in areas previously occupied by savanna vegetation. These results and other published data for the Amazon region indicate that the areas representing today's forest-savanna boundaries have been determined by significant vegetation changes during the Holocene. The boundary between forest and savanna vegetation seems to be quite sensitive to climatic change and should be the focus of more extensive research to correlate climate and past vegetation dynamics in the Amazon region.
... During the Pleistocene there were about 21 major fluctuations in temperature, precipitation, atmospheric composition and hydrology, this dynamic environment being reflected by changes in species distributions. The most recent of these fluctuations, characterised by a maximum drying and cooling of the climate, occurred about 21 000 radiocarbon years before present ( 14 C yr BP) and ended about 10 000 14 C yr BP (Hamilton 1982;Bonnefille et al. 1990;Servant et al. 1993). Species responded to climatic change by exhibiting three responses: acclimatisation, adaptation or migration, with a species having to respond in one way, or a combination of ways, to persist. ...
The paleoclimate archives of the tropics and marginal tropics from Mexico to southern Brazil and their interpretation are presented and critically discussed. The importance of Quaternary glaciations for climate reconstruction is particularly emphasized and related to Quaternary environmental changes in the Central and South American lowlands. Older glacier traces, often dated much too young, provide evidence of thermal and hygric climatic fluctuations since the early Quaternary that were much greater than the climatic changes of the last ice age. The maximum glaciation of the Little Ice Age occurred in the Southern Hemisphere around CE 1700, north of the equator around CE 1850. Cold phases are characterized by greater aridity in the lowlands. The orbital and solar influence on large and small climate variations is documented by many different paleoclimate archives. Terrestrial South American paleoclimate data are correlated predominantly with Arctic and less with Antarctic data. Climate modeling rarely considers the terrestrial paleoclimate data of the New World tropics. The extent to which climate influenced human cultures is discussed using Central America as an example and is further elaborated in Chap. 9.
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The South Atlantic Convergence Zone (SACZ) profoundly modulates precipitation from central to southeastern Brazil in the present‐day climate. However, the understanding of its long‐term behavior responding to various climate forcings remains limited. Here, we use an isotope‐enabled atmospheric general circulation model (ECHAM4.6) to examine the precipitation response of the SACZ during the mid‐Holocene about six thousand years ago. The model simulates a northward intensification of the SACZ in the mid‐Holocene, resulting in a dipole anomaly pattern relative to today's climate. The mid‐Holocene precipitation increased along the northern margin of the SACZ due to the strengthening of easterly winds across the tropical Atlantic, while an eastward deflection of the South American low‐level jet reduced moisture transport to southern Brazil, resulting in reduced precipitation along the southern margins of the SACZ. The north–south dipole response in precipitation is consistent with the mid‐Holocene hydroclimate change observed in proxy records from the region.
Phylogeny is a potentially powerful tool for conserving biodiversity. This book explores how it can be used to tackle questions of great practical importance and urgency for conservation. Using case studies from many different taxa and regions of the world, the volume evaluates how useful phylogeny is in understanding the processes that have generated today's diversity and the processes that now threaten it. The novelty of many of the applications, the increasing ease with which phylogenies can be generated, the urgency with which conservation decisions have to be made and the need to make decisions that are as good as possible together make this volume a timely and important synthesis which will be of great value to researchers, practitioners and policy-makers alike.
Lagoa Santa is one of the most important places in South America in the realm of natural history knowledge. Its renown as the birthplace of Brazilian paleontology is due to the research first undertaken there by Peter Lund in the 19th century. The fauna of the Lagoa Santa Karst is notable for the presence of typical Cerrado savannah species and of the Atlantic Forest domain. Zoologically, the region is extremely important insofar as, in addition to sheltering a peculiar fauna of its own, it is one of the few areas of Brazil for which meticulous inventories have been published. That historical data has made it possible to understand the composition of the groups and to evaluate natural and anthropic-induced changes that have occurred in the course of the past 200 years. Based on an analysis of recent studies, it has been shown that the vertebrate fauna in the region is composed of 70 species of fish distributed among 22 families; 25 species of amphibians belonging to 9 families, and 41 species of reptiles in 14 families. There are also 240 bird species belonging to 58 families and the mammal fauna consists of 107 species belonging to 28 families. In comparison with the historical records of species, there has been a serious reduction in numbers. Evidently, the most important explanatory factor for that reduction is Man’s action in deteriorating the natural environment.
This work presents and discusses some characteristics of the deep indigenous history of Monte Castelo, a southwestern Amazonian shellmound site, in the light of recent research on that site and the archaeology of shellmounds throughout the region. The data obtained at Monte Castelo confirm that the oldest and most persistent ceramic assemblages in the Americas are located in shellmounds, in contexts where the construction of the landscapes has lasted for millennia, marking periods of intensification in the human occupation of the Amazon Basin. Material culture, stratigraphy and chronology are presented in order to characterize the fundamental traits relating to the origin and development of ceramic technology and landscape management in the lowlands of South America. Human intervention in the landscape has long provided for the reoccupation of many of the earliest known archaeological sites. Parallel to this, several paleoenvironmental markers in the southern Amazon have evidenced variations in the climate that
accompany human occupations since, at least, the Early Holocene. In the Guaporé river basin, the chronology of the sites seems to accompany trends of increased water availability and forest expansion, in a period marked by the emergence of more
numerous communities and complex artifacts throughout the Middle Holocene. There, the feedback between human interventions and climate change has created a privileged place for settlements, whose striking relative continuity has given rise to some of the most important cultural and landscape changes that have spread widely throughout the Amazon and beyond for thousands of years. Seeking to bring the notion of meaningful places to the archaeology of the Amazonian shellmounds, this
work proposes a way to understand them through an inclusive notion of ancestry that may be useful for contemporary indigenous peoples to recover their traditional
territories.
The genus Temnorhynchus Hope (Insecta: Coleoptera: Scarabaeidae: Dynastinae) is distributed in the Afrotropical and Malagasy regions, Arabia and the East-Mediterranean. The historical development of present distribution areas (range systems) of some taxa of these rhinoceros beetles is discussed, considering autecological and dispersal characteristics of the species and data on fluctuations of climate and vegetation cover. Some phylogenetic branching events of Temnorhynchus are correlated with climatic changes of the late Quaternary in Africa, especially with the Last Glacial Maximum (around 18,000 b.p.) and the preceding pluvial (around 28,000 b.p.). The environmental change in the Holocene Altithermal (around 8,000 b.p.) influenced the range systems of extant species. Biogeographical analysis leads to an improvement of the hypothetical phylogenetic system and also to some taxonomic consequences: T. coronatus (Fabricius) and T. baal Reiche & Saulcy form the T. coronatus-superspecies. T. burgeoni Paulian, T. minor Paulian, T. congoanus Paulian, T. raffrayi Fairrnaire, and T. zambezianus Krell form the T. raffrayi-superspecies. T. retusus (Fabricius), T. clypeatus Klug, and T. elongatus Arrow form the T. retusus-group.
The Mid-Holocene (6 ka BP) is a key period to the study of climate, since it presented lower than present incoming summer insolation in the Southern Hemisphere, and the opposite in the Northern Hemisphere. This happened due to a different than present configuration of the orbital parameters. To investigate the effects of insolation on the Mid-Holocene climate, some global and regional multiproxy palaeodata compilations have been elaborated. However, few global studies have focused on the Southern Hemisphere, and none of the regional ones have characterized the Mid-Holocene climate in South America through a multiproxy approach. Here we present the first multiproxy compilation to the Mid-Holocene climate in eastern South America. We have compiled 120 palaeoclimatological data, published in 84 different papers. The palaeodata analyzed suggest a water deficit scenario in great part of eastern South America during Mid-Holocene, except for Northeastern Brazil. Nonetheless, further sampling is mandatory in South America and in the adjacent ocean basins.
CO2 and carbon cycle changes in the land, ocean and atmosphere are investigated using the comprehensive carbon cycle-climate model NCAR CSM1.4-carbon. Ensemble simulations are forced with freshwater perturbations applied at the North Atlantic and Southern Ocean deep water formation sites under pre-industrial climate conditions. As a result, the Atlantic Meridional Overturning Circulation reduces in each experiment to varying degrees. The physical climate fields show changes that are well documented in the literature but there is a clear distinction between northern and southern perturbations. Changes in the physical variables affect, in return, the land and ocean biogeochemical cycles and cause a reduction, or an increase, in the atmospheric CO2 by up to 20 ppmv, depending on the location of the perturbation. In the case of a North Atlantic perturbation, the land biosphere reacts with a strong reduction in carbon stocks in some tropical locations and in high northern latitudes. In contrast, land carbon stocks tend to increase in response to a southern perturbation. The ocean is generally a sink of carbon although large re-organizations occur throughout various basins. The response of the land biosphere is strongest in the tropical regions due to a shift of the Intertropical Convergence Zone. The carbon fingerprints of this shift, either to the south or to the north depending on where the freshwater is applied, can be found most clearly in South America. For this reason, a compilation of various paleoclimate proxy records of Younger Dryas precipitation changes are compared with our model results.
Pedological and palynological data from Eneolithic and Bronze Age settlements in the steppe zone of eastern Ukraine show that the transition from the mild climate of the Atlantic period (6,500–5,500 BP) to the more continental climate of the Subboreal was marked by considerable impoverishment of the floristic composition and by reduction or disappearance of several mesophilic and thermophilic taxa. Within this general background, there were also rhythmic oscillations between wet climate stages marked by expansion of forest-steppe, with dry climate stages marked by expansion of steppes. Five such shifts have occurred between 5,500 and 2,500 BP. The strongest aridification occurred between about 4,100 and 3,500 BP. It is marked by the decrease of humus accumulation, biogenic activity and chemical weathering in the soils, which became rather loess-like and contain deep desiccation fissures. Pollen data show sharp reduction of forest areas, significant xerophytization of steppe vegetation, and drying of flood plain swamps. The forest-steppe landscapes of the Early Subboreal were replaced by Artemisia-Gramineae steppes representing the shift through three phyto- geographic subzones. None of the Holocene climatic fluctuations was as sharp and contrasting when compared with the preceding and following stages. In studied regions, no major paleoenvironmental changes were observed within that interval that could be clearly ascribed to human impact. The Mid-Subboreal aridification is therefore considered to be of natural origin.
Peters and Darling (1985), in a pioneering paper on The Greenhouse Effect and Nature Reserves, singled out island species as a special case of geographically restricted species which are particularly vulnerable to climate change — if the latitudinal or altitudinal migration required by the climate change exceeds the limits of the island, extinction necessarily follows. They recognized a mitigating circumstance, however, in that climatic changes on oceanic islands might be relatively mild because the sea would moderate the air temperature change.
The salt lake of Sua Pan, one of the largest salt lakes in Botswana, is important resources of natural alkali. To identify the sources and genesis of subsurface brine in this salt lake, groundwater and surface water near this lake, in addition to subsurface brine water in the lake and salt crystals from solar salt field were collected and determined. The contents of major cation and anion, the isotope compositions of D, 18O and 37Cl, and the 14C age of groundwater were calculated. It is found that the subsurface brine is characterized by enriched Na and K, and depleted Ca and Mg. Results of D, 18O and 37Cl show that surface water is closely connected with subsurface brine in Sua Pan, while the role of groundwater recharge on it is weak (the difference of 37Cl is 0.04‰-0.06‰). Relationships of TDS-γNa/γCl indicate that leaching of halite affects the formation of this subsurface brine (γNa/γCl≈1), and the age of 14C (about 20 000 years ago)indicates that the fluctuation of ancient climate is considered to be an important factor of the formation of Sua Pan. Based on the above knowledge, inverse simulation of subsurface brine in Sua Pan is modeled by PHREEQC software, which further verifies that salt lake of Sua Pan is mainly the result of strong evaporation and concentration of surface water and halite leaching by groundwater.
Carbon and oxygen isotopes, organic matter elemental composition, and mineralogy of carbonates were used as proxies to investigate environmental changes recorded in the carbonate-rich sediment of the hypersaline lagoon "Vermelha", Rio de Janeiro. Isotopic and other data suggest that two striking shifts in environmental conditions occurred about 1900 and 3200 years ago. Carbonates δ(18O) and δ(13C) were found in the range of +1.5 ‰ to +4.5 ‰ (PDB), and -2 ‰ to -11 ‰ (PDB), respectively. In organic matter, δ(13C) ranged from -22 ‰ to -15 ‰ (PDB). Calcites and dolomites are present in the studied core; their relative abundance seems to be related to the observed changes. Calcites are enriched up 38 % in MgCO3 (mole fraction, x). As distinct from calcites with lower magnesium contents, Mg-calcite with x(Mg) > 20 % shows an additional peak at 0.2929 nm in the X-ray diffractogram. Samples that contain Mg-calcite or 100 % dolomite show similar δ( 13C) values. There are strong indications for an early formation of dolomite already in the microbial mat region. 18O and 2H determinations in lagoon water confirm evaporation as the water balance controlling process.
Tropical montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastal regions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive 2000-3000. mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfalls that alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic landforms that can be solely attributed to their low-latitude locations. Whereas they are relatively understudied, an emerging view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a high frequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to transport finer-grained sediment also suggest that the restorative processes in these systems Elsevier Inc. All rights reserved.
Brush-finches of the genus Atlapetes present a complicated pattern of differentiation along the Andes. Forms with different colours may replace each other sharply at different altitudes on the same slopes, in similar humid habitats on different slopes or on adjacent humid and semi-humid slopes. In the current classification, grey and yellow-and-green species are placed in different groups, and similar-coloured populations replacing each other in corresponding ecological zones on different slopes are ranked as subspecies of the same species. However, Remsen and Graves have suggested that different-coloured forms inhabiting adjacent mountains could be representatives of the same species with different pigment saturations, rather than competing species. The relationships between and within Atlapetes species are assessed based on mtDNA sequence data. Three main branches are demonstrated, comprising southern (Bolivian, northwest Argentinean), central (south and central Peruvian) and northern/western species. Clearly, the traditional groupings based on plumage colours did not demonstrate evolutionary trajectories in the genus. The data also show that many forms, currently ranked as subspecies, are genetically more divergent than sympatric species. Species rank is suggested for several forms hitherto treated as subspecies of geographically disjunct species.
In Bolivia, polar advections are non periodic phenomena, four times more frequent in winter than in summer. Their pluviometric and thermal effects differ from one season to another and are mainly noticeable in the Andean valleys characterized by a south-east direction and in the plain. In the latter, the polar advections are felt within a latitude as low as 10 degrees. -English summary
Palynological investigations on sediment cores from three localities in Rondonia in the southern part of the Amazon Basin, indicate that marked vegetational changes have ocurred there. The series of samples from Katira represents the late Cenozoic, probably Quaternary. The sediments from Capoeira might be partly of Holocene age (and possibly Upper Pleistocene as well). Apparently the climatic changes during several intervals of the Late Cenozoic (Quaternary) caused the development of savannas in this region which is now covered in tropical forest.
In recent years it has generally been accepted that Amazonia was subject to long dry periods in the late Pleistocene and post-Pleistocene which induced forest cover to a few limited areas or refuges. It has been proposed that the subsequent genetic isolation into separate populations is a mnjor factor in the evolution of the species diversity within the lowland forest of Amazonia. Most of the previous evidence for this theory is based on studies of animals, for example: lizards, butterflies, and birds. Here data are presented to confirm the theory of forest refuges using evidence from phytogeography. Distribution patterns of the lowland species of the woody plant families Caryocaraceae, Chrysobalanaceae, Dichapetalaceae and Lecythidaceae are discussed and concur with the possibility of forest refuges. A map is given of the refuge areas that seem most likely, based on evidence from species distribution of the above plant families. The refuges proposed here correspond closely with the refuge areas proposed by Haffer and Brown rather than the extremely reduced areas proposed by Vanzolini.
EVIDENCE from ice cores1 indicates that concentrations of
atmospheric carbon dioxide were lower by about 75 p.p.m. during the Last
Glacial Maximum (LGM; ~18,000 years ago) than during the present
interglacial (10,000 years ago to the present). The causes of such large
changes in atmospheric CO2 remain uncertain. Using a climate
model, Prentice and Fung2 have estimated that there was
approximately the same amount of carbon in vegetation and soils during
the LGM as there was during the present (pre-industrial) interglacial.
In contrast, we present here results based on palynological, pedological
and sedimentological evidence which indicate that in fact the amount of
carbon in vegetation, soils and peatlands may have been smaller during
the LGM by ~1.3x 1012 tonnes. Thus, organic carbon in
vegetation and soils has more than doubled (from 0.96 to 2.3 x
1012 tonnes) since the LGM. Oceanic CO2 reservoirs
seem to be the only possible source of this large quantity of carbon
that has entered the terrestrial biosphere since the LGM (in addition to
that which has entered the atmosphere to give the higher interglacial
CO2 levels).
Changes in solar radiation arising from changes in the orientation of
the earth's axis had pronounced effects on tropical monsoons and
mid-latitude climates as well as on ice-sheet configuration during the
last 18,000 years. COHMAP (Cooperative Holocene Mapping Project) has
assembled a global array of well-dated paleoclimatic data and used
general-circulation models to identify and evaluate causes and
mechanisms of climatic change. Comparisons of paleoclimatic data with
the model simulations are important because models provide a theoretical
framework for evaluating mechanisms of climatic change, and such
comparisons help to evaluate the potential of general circulation models
for predicting future climates.
THERE has been considerable debate about the magnitude of the decrease in temperature1,2 and the change in precipitation3 in the African tropics during the last glacial period. With the advent of fossil pollen studies in equatorial regions, it is now generally agreed that the temperature did decrease at this time in tropical regions4, but the magnitude of the temperature fluctuations and discrepancies between the continental5 and marine6 temperature records have yet to be resolved7. Here we present new quantitative estimates of temperature and precipitation using a multivariate analysis8 of pollen time-series data from peat deposits in Burundi for the past 40,000 years9. For the last glacial period, our estimate of a temperature decrease of 4 +/- 2 °C is less than those (ranging from 5 to 8 °C) derived from snow-line and tree-line records5,7. Model simulations7 indicate that the snow-line and tree-line estimates (from high-elevation sites at ~4,000 m above sea level) are incompatible with the marine temperature record. Our lower estimate from a site of intermediate elevation may help resolve the differences between these records. We also estimate that the mean annual rainfall decreased by 30% during the last glacial period, in agreement with the rainfall history inferred from lake level fluctuations10.
During present-day strong El Nino events, climatic anomalies appear in different regions of South America. The same climatic mode has been observed in the Holocene in the same regions. A conceptual model, explaining these climate anomalies during the Holocene by long-term El Nino-like conditions, is proposed. There is an abridged English version. -English summary
Mean annual paleotemperatures are estimated from the altitudinal shifts of fossil diatom assemblages, compared with their present altitudinal zonation. The results suggest that temperature was close to the present one between 9000 and 5000 yr BP. From 5000 years BP the climate was colder than present, with large amplitude oscillations. There is an abridged English version. -English summary
The dense Amazonian forest decayed and disappeared in the area of Santa Cruz de la Sierra (Bolivia) during time intervals 7000-5000 and 3400-1400 BP. The climate of Holocene period fluctuated with a periodicity of about 1000-2000 yr in this part of the Amazonian basin.-English summary
Palynological studies in the Northern Andes have shown a gradual upheaval of the Cordillera during the Late Pliocene and the creation of the high montane environment. A long sequence of glacial and interglacial periods has been recorded from the Pleistocene. The successive appearance of new taxa, by evolutionary adaptation from the local neotropical flora and from elements immigrated from the holarctic and austral-antartic floral regions, can be followed step by step. For the Last Glacial to Holocene sequence the contemporaneity of the changes of temperature with those recorded from the northern temperate latitudes could be proved by 14 C dating During the coldest part of the Last Glacial the tree line descended to c. 2000 m altitude, i.e. 1200-1500 m lower than where it lies today. During the period from c. 21,000 to c. 13,000 B.P. the climate was, moreover much drier. Even taking the greater aridity into account, the lowering of the temperature during the coldest part of the Last Glacial may have been 6-7⚬ C or more. The lowering of the temperature in the tropical lowlands during glacial times may have been c. 3⚬ C. The temperature gradient must, therefore, have been steeper than it is today. In the coastal lowlands of Guyana and Surinam glacial-interglacial eustatic movements of sea level have been recorded. Pollen diagrams show in this area a considerable extension of savannas during glacial periods with low sea levels. In the inland savannas of the Llanos Orientales of Colombia and the Rupununi savanna of Guyana, several periods of grass-savanna and of savanna-woodland alternate during the Late Pleistocene and the Holocene; lower and higher lake levels corroborate the conclusions that these are caused by changes in the effective precipitation. One of the driest periods in the Rupununi seems to correspond to the time immediately before c. 13,000 B.P. Pollen data a series of samples from Rondonia, in the southern part of the Amazon basin, have shown that in that area grass-savannas replaced the tropical forest during a certain interval of Pleistocene age. From the above it appears that in considerable parts of the South American tropics a much drier climate prevailed during certain parts of the Pleistocene. A major dry period seems to have occurred during the later part of the Last Glacial, when the glaciers in the northern latitudes and in the Andes were reaching their maximum extension. These changes of climate and vegetation are of considerable importance for the explanation of speciation patterns and the recent distribution of plant and animal taxa.
The explosion crater (maar) Lake Kumpaka in the western Amazon basin of Ecuador has yielded an 18.6 m sediment core spanning 5200 radiocarbon years. Sedimentary stratigraphy and pollen analysis provide the first record of rain forest climate and vegetation at a site undisturbed by riverine process. Deeper sediments were deposited very rapidly, apparently as a result of a 500-year episode of drier climate that led to massive slumping or rapid erosion about 4000 years ago. A complex history of local storms is preserved through the upper part of the record as textural banding. A three-zone pollen history is recognized, with boundaries at 3300 and 900 years BP. All three pollen zones are taken to represent facies of intact tropical rain forest. A large influx of pollen of the colonizing trees is present at all intervals, but the remaining pollen is of a diverse array of rain forest trees. About 350 pollen taxa are recognized, but only about 100 can be named. Changes in the pollen diagram synchronous with the postulated flooding event of northern Ecuadorian Amazonia 1300-800 years ago are apparent, but the palaeoecological significance of the changes cannot be assessed satisfactorily with the present data. It is possible that the regime of high precipitation in northern Ecuador was contemporaneous with a drier climate at Kumpaka, suggesting that the two sites may be on opposite sides of a climatic divide. The pollen record demonstrates that a complex history of Amazonian rain forest is preserved in lake sediments, despite the prevailing animal pollination mechanisms.
Data presented for the mid and late Holocene indicate that effective moisture has varied, but that climates were never again as mesic as the early Holocene. In general, arid conditions prevailed except from 5200 to 2200 BP. The vegetational record begins (c13 000 BP) with savanna surrounding a saline marsh. The watershed near the lake was not part of a refugium for mesophytes. As the Holocene began, halophytic littoral and shoreline communities formed around a shallow, saline lake. The coverage of savanna declined and contained scattered scrub. These communities were replaced (c9800 BP) by a dry, lower montane forest indicating cool temperatures. These forests were soon replaced (c9500 BP) by semievergreen and deciduous subtropical forests with Brosimum predominant. Over the past 8300 yr essentially modern vegetational associations existed within the watershed. Mesic forests retreated to higher elevations during the mid-Holocene as forests at lower elevations became more open and deciduous. Extensive weedy vegetation persisted on the unstable flood plains. Forests expanded somewhat 5200-2200 yr ago but have since declined in extent. -from Author
The geomorphic events that formed the west Mexican coastal plain and the stratigraphy and chronology for those events based on soils, pollen zones, and radiocarbon ages are discussed. Three phases of geomorphic development are recognized. The Alluvial Phase represents deposition of the coastal bajada during the Neogene; oxisols and saprolites have formed on this surface. The Teacapan Peninsula Phase began during the Sangamonian Interglacial with deposition of the spit; an ultisol and an alfisol have developed on the peninsula. In the Prograding Beach Phase, beginning about 13,000 B.P., four sequences of beach ridges were deposited during eustatic sea level rise and periodic uplift. On the resulting ridge and swale topography are four soil units, three inceptisols on the initial, early, and middle beaches and an entisol on the late beach. The pollen stratigraphy and chronology are based on samples from eleven cores, twelve radiocarbon dates, and calculated deposition rates. The alternation in dominance of mangrove pollen taxa records relative changes in sea level due to tectonic uplift and sea level rise. The data record filling of the lagoon behind the Teacapan Peninsula beginning prior to 8,000 years ago during eustatic sea level rise. A mangrove pollen association that corresponds to this rise continues into the mid-Holocene. A subsequent Avicennia rise indicates a relative lowering of sea level possibly due to uplift of the coastal plain. Between 7,000 and 6,000 B.P., increases in pine and oak indicate expansion of upland forests during a cooler climatic interval. A marine incursion around 5,650 B.P. documents a peak in sea level rise. In late Holocene time, marsh taxa become more important as beaches prograde westward and the lagoons receive more freshwater runoff. After 5,000 B.P., upland forest taxa again increase. The most recent, Avicennia-dominated associations again show lower sea level or uplift. Late Holocene pollen zones include one of nonarboreal pollen dominance and lastly one enriched in pine, nonarboreal and lowland forest taxa, and cultivated plants. The pollen data support age relationships inferred in the Prograding Beach Phase. Just as all of the beaches are represented in the Teacapan area of the coastal plain, the oldest pollen record occurs in the lagoon behind the Teacapan Peninsula. Higher sea levels are indicated by red mangrove prominence for the Initial Beach and by the Dictyocha zone for the Early Beach. The southern part of the coastal plain has only the younger beaches and only the younger Holocene pollen zones in the lagoonal records.
The deepest part of a core from Lake Valencia (403 m elevation) was studied. Radiocarbon dating shows that the section includes the Pleistocene—Holocene boundary. The pollen analysis indicates that the Pleistocene lake had desiccated, and from 13,400 to approx. 11,500 B.P. the site (today under 40 m water) was a swamp or intermittent lake. The region was covered by semi-arid vegetation. Shortly before 10,700 B.P. precipitation increased but evaporation was probably very high. Around 10,000 B.P. the lake started to form again. Dry or thorn forest and savannas occupied the region around the lake, and rain forest covered the mountain top. At that time the lake was smaller than today.The Valencia dry phase at the end of the Pleistocene corresponds to the end of the Glaciation period in the northern Andes.
Surface observations for the 10-year period 1961-70 are analyzed for occurrences of fronts in Northeast Brazil. Cold fronts, or their remains, are found to enter the Northeast throughout the year. The southern part of the region (interior Bahia) receives much of its precipitation in the period November-February associated with frontal systems. Coastal areas, which receive maximum rainfall during the months of May-July, experience a marked increase in rainfall associated with the approach and passage of cold fronts. Even precipitation events in the extreme northern part of the region (Cearea) sometimes appear to be associated with systems which are cold-frontal in origin.- from Author
In Laguna Chichancanab, the largest closed basin in northern Yucatan, large-scale changes in lake levels have been documented by "0 analyses of snail shell carbonates. A continuous 9-m series of lake sediments has been deposited during the last 8,000 years, whereas a discontinuous sedimentary record extends from 9 m to beyond 12 m in depth. At least one marked hiatus occurred in this older record during which time the lake is thought to have been seasonally dry (or very much reduced in size). The fluctuations are verified by other stratigraphic evidence including absolute numbers of shells, loss on ignition, carbon:nitrogen ratios, and major cations (calcium, magnesium, and sodium). We here consider a new approach to determine lake level stability in tropical, closed lake basins where mean annual tem- peratures have remained relatively stable in comparison with changes in rates of inflow and evaporation. The general goal is to document long term changes in the lake's water balance and to relate this in- formation to population dynamics of aquatic organisms, particularly benthic molluscs living in the littoral zone. We use "closed lake" to mean a lake which receives inflow from precipitation and its drainage basin but loses water only by evaporation. Many previous studies on lake level stability ( see Richardson 1969 for a review ) have emphasized the need to correlate several independent lines of evidence. This iso- topic analysis is another direct approach which can yield unique information un- available from other types of analyses of closed, tropical lakes. Naturally occurring isotopes of 180 and 14C can be considered tagged atoms and their movements monitored for the long term study of water balance. These iso-
A continuous deuterium record along the Vostok ice core has been
obtained and is interpreted in terms of local surface temperature
changes over the past 160 kyr. The record is dominated by the large
glacial-interglacial signal occurring at about 100 kyr with a total
temperature amplitude of about 11 C. It is confirmed that the warmest
part of the Last Interglacial Period was about 2 C warmer than the
Holocene. This climatic record is the awaited terrestrial complement of
the deep-sea records supporting the existence of a relation between the
Pleistocene climate and orbital forcing.
During mid-July 1975, a strong frontal system crossed into the tropical
latitudes of South America. Freezing temperatures were reported in
southern Brazil, and the daily thunderstorm activity over the Amazon
River Basin gave way to low stratiform cloudiness. Daily infrared data
from the SMS-1 satellite are presented for this weather event.
CLIMAP (1981, “Seasonal Reconstruction of the Earth's Surface at the Last Glacial Maximum,” Geological Society of America Map and Chart Series MC-36) boundary conditions were used as inputs to the GISS general circulation model, and the last glacial maximum (LGM) climate was simulated for six model years. The simulation was compared with snow line depression and pollen-inferred temperature data at low latitudes, specifically for Hawaii, Colombia, East Africa, and New Guinea. The model does not produced as much cooling at low latitudes as is implied by the terrestrial evidence. An alternative experiment in which the CLIMAP sea-surface temperatures were uniformly lowered by 2°C produces a better fit to the land data although in Hawaii model temperatures are still too warm. The relatively warm CLIMAP tropical sea-surface temperatures also provide for only a slight decrease in the hydrologic cycle in the model, in contrast to both evidence of LGM tropical aridity and the results of the experiment with colder ocean temperatures. With the CLIMAP sea-surface temperatures, the LGM global annual mean surface air temperature is 3.6°C colder than at present; if the ocean temperatures were allowed to cool in conformity with the model's radiation balance, the LGM simulation would be 5°–6°C colder than today, and in better agreement with the tropical land evidence.
The isotopic stratigraphy of piston cores from the upper fan of the Amazon Cone reveals a negative planktic foraminiferal δ18O deviation from the global oxygen isotope signal that extends back into the last deglacial period. The benthic foraminiferal δ18O signal agrees with the global oxygen isotope signal and supports the lithological interpretation that the glacial/interglacial boundary occurs at the iron-rich crust on the Amazon Cone. The negative planktic foraminifera δ18O deviation from the global δ18O signal is interpreted as evidence of increased deglacial Amazon freshwater discharge to the equatorial Atlantic Ocean. 14C dates suggest that the Amazon tropical freshwater discharge initiated at 13.5 ky B.P., peaked at 9.6 ky B.P., and terminated at approximately 6 ky B.P. The timing of the tropical meltwater spike (Showers and Margolis, 1985) which peaked in the southern Venezuelan Basin at 9.6 ky B.P. can be linked to the flooding of the Amazon shelf and the reinitiation of the Brazilian Coastal Current. This change in equatorial circulation and advection of Amazon River freshwater to the North Atlantic via the Caribbean Surface Current, the Gulf of Mexico-Loop Current, the western Atlantic-Gulf Stream and the North Atlantic Drift may be the cause for the rapid termination of the Younger Dryas cool period.
A new climatic time series has been determined from delta O-18 analysis
of a 2083 m ice core drilled at Vostok Station. The site is located on
the high Antarctic Plateau, where the ice thickness is about 3700 m. The
isotope profile covers the last 150 kyr, back to the ice age which
preceded the last interglacial, and has essentially been undisturbed by
flow conditions. The results indicate that the present Holocene was
preceded by a long glacial period marked by two relatively warm
interstadials. The well marked last interglacial was significantly
warmer than the Holocene and the end of the previous glacial was quite
similar to the last glacial maximum. The presence of a roughly 40 kyr
cycle in the Vostok record strongly supports the role of orbital forcing
in determinig climatic change.