Vegetation history and climate changes in West Equatorial Africa during the Late Pleistocene and Holocene, based on a marine pollen diagram from the Congo fan

Palynological investigation of the marine core, GeoB 1008-3, from near the mouth of the Congo river (6°35.6S/10°19.1E), provides information about the changes in vegetation and climate in West Equatorial Africa during the last 190 ka. The pollen diagram is divided into zones 1–6 which are considered to correspond in time with the marine isotope stages 1–6. Oscillations in temperature and moisture are indicated during the cold stage 6. During stage 5, two cooler periods (5d and 5b) can be shown with an expansion of Podocarpus forests to lower elevations on the expense of lowland rain forest. Extended mangrove swamps existed along the coast in times of high sea level (stages 5 and 1).     

Vegetation history of the western Rif mountains (NW Morocco): origin,late-Holocene dynamics and human impact

Unlike southern Spain, northern Morocco has been little investigated for palaeoecological purposes. Consequently, the origin and history of the Rifan vegetation is largely unknown, as well as the past role of human activities. A review of the Plio-Pleistocene fossil data available from North Africa clearly reveals the ancient origin of much of the present-day flora and vegetation structures of the region. A well-dated pollen record covering the last 5,000 years, obtained from a fen, is compared to previous regional pollen data in order to understand the late-Holocene vegetation dynamics and the influence of anthropogenic disturbances. Modern pollen spectra have allowed the calibration of pollen diversity and evenness as indicators of tree-cover density. The results obtained show the long-term persistence of regional forests until the onset of Arab Sharifian dynasties in the 16th century, with a surprising lack of human impact during the late Neolithic, and little impact during Roman colonisation. The increasing density of deciduous forests recorded from 3,800 to 1,900 cal bp, concomitant with the expansion of cedar in the Middle Atlas, could reveal the onset of widespread cooler and moister climatic conditions. The weak and late human impact in the Rifan mountains explains, at least in part, their high diversity and the conservation of their forest ecosystems. Anthropogenic activities however have led to severe modification of the understorey structure of these forests during the four last centuries, and to the worrying on-going deforestation that presently threatens the survival of this invaluable biological heritage.     

The Holocene history of low altitude Mediterranean Fagus sylvatica forests in southern France

Questions

As the dominant tree in many European forests, Fagus sylvatica functions as an ecosystem engineer, yet its istory remains little understood. Here we ask: (a) are there indications for its presence in southeast France during the last Glacial period; (b) what was the timing of the expansion and decline of F. sylvatica dominated forests; (c) which factors influenced their dynamics and in particular to what extent did past precipitation changes impact upon them; and (d) at which altitudes did these beech forests occur within the region?

Location

Languedoc, the French Mediterranean area.

Method

This article presents a well dated and high‐resolution pollen sequence covering the last 7,800 years from the Palavas Lagoon in the Languedoc together with a review of Fagus charcoal occurrences in the Languedoc and the lower Rhône Valley, and a review of pollen data from a compilation of 69 sites in southeast France.

Results

The Palavas pollen sequence provides a regional summary of F. sylvatica abundance changes near the Mediterranean coast. Around 6,000 years cal BP , an abrupt transition from small beech populations to well‐developed forests is recorded. The maximum development of beech forests occurred between 4,000 and 3,000 years cal BP , while F. sylvatica started to regress after 3,000 years cal BP .

Conclusion

Scattered F. sylvatica populations probably survived throughout southern France during the last Glacial period. F. sylvatica started to spread around 8,000 years cal BP while beech forests never expanded before 6,000 years cal BP . The complex patterns of F. sylvatica expansion in southern France after 6,000 years cal BP suggests that a combination of global (climate change) and local (human impact) factors were responsible for this major change. Recurrent abrupt climate changes, the aridity trend and human deforestation caused beech forests to decline after 3,000 years cal BP .

     

On the holocene history of vegetation in SE Bulgaria (Lake Arkutino,Ropotamo region)

Palynological studies and investigations on macrofossil remains have been carried out on two profiles of the Holocene sediments of Lake Arkutino situated near to the Black Sea coast of SE Bulgaria. Lake Arkutino is a freshwater lake with shallow water. The lake is separated from the Black Sea by a dune barrier. Sedimentation started at about 6100 B.P. No marine or brackish influence could be traced in the sediments although the lake level is around sea level. The history of vegetation does not display major fluctuations except for the lake vegetation itself and for the swamp forests (Longos forests) surrounding the lake. Forests on normal mineral soils were mainly occupied by deciduous oaks throughout the last 6100 years. Deciduous oaks are represented in the pollen diagrams by the Quercus robur type and the Quercus cerris type. Until about 5800 B.P., one of the two pollen diagrams indicates that forests still contained more Pinus and Corylus than afterwards. After 5800 B.P., Fagus and a hundred years later together with Carpinus betulus became more important in addition with submediterranean elements as Carpinus orientalis, Fraxinus ornus and Phillyrea. After 5200 B.P., Carpinus betulus became somewhat less important. Indicators for human influence are rare. The archaeological record speaks for a small population in the area under study since the 12th century B.C. Swamp forests of the so-called Longos forests type started to develop at about 3000 B.P. The macrofossil record offers possibilities to trace hydrological changes by tracing the history of hydrophyte and reed plant comminties. From about 5100 to at least 4000 B.P., the record for Lemnetea and Potametea communities is poor and the processes of terrestrialization was interrupted. It can be assumed that the highest Black Sea transgression which is dated to 5700–4000 B.P. and which showed a sea level 3–4 m higher than today heavily influenced Lake Arkutino. That may have resulted in a higher lake level and in some water supply from the nearby river Ropotamo which would explain certain changes in the sediments and in the composition of hydrophyte communities.     

Response of tundra ecosystem in southwestern Alaska to Younger-Dryas climatic oscillation

Climatic warming during the last glacial–interglacial transition (LGIT) was punctuated by reversals to glacial‐like conditions. Palaeorecords of ecosystem change can help document the geographical extent of these events and improve our understanding of biotic sensitivity to climatic forcing. To reconstruct ecosystem and climatic variations during the LGIT, we analyzed lake sediments from southwestern Alaska for fossil pollen assemblages, biogenic‐silica content (BSiO₂%), and organic‐carbon content (OC%). Betula shrub tundra replaced herb tundra as the dominant vegetation of the region around 13 600 cal BP (cal BP: ¹⁴C calibrated calendar years before present), as inferred from an increase of Betula pollen percentages from << 5% to >> 20% with associated decreases in Cyperaceae, Poaceae, and Artemisia. At c. 13 000 cal BP, a decrease of Betula pollen from 28 to << 5% suggests that shrub tundra reverted to herb tundra. Shrub tundra replaced herb tundra to resume as the dominant vegetation at 11 600 cal BP. Higher OC% and BSiO₂% values suggest more stable soils and higher aquatic productivity during shrub‐tundra periods than during herb‐tundra periods, although pollen changes lagged behind changes in the biogeochemical indicators before c. 13 000 cal BP. Comparison of our palaeoecological data with the ice‐core dδ¹⁸O record from Greenland reveals strikingly similar patterns from the onset through the termination of the Younger Dryas (YD). This similarity supports the hypothesis that, as in the North Atlantic region, pronounced YD climatic oscillations occurred in the North Pacific region. The rapidity and magnitude of ecological changes at the termination of the YD are consistent with greenhouse experiments and historic photographs demonstrating tundra sensitivity to climatic forcing.     

Upland vegetation in the north-west Iberian peninsula after the last glaciation: Forest history and deforestation dynamics

This paper presents the results of pollen analyses from organic sediments of seven cores (299 spectra) in a mountainous area of the north-west Iberian peninsula. The pollen diagrams, supported by seven¹⁴C dates, are used to construct a regional pollen sequence covering the main stages of vegetation dynamics, from the last phases of the Late-glacial until the present. During the Late-glacial Interstadial an important development of cryophilous forests (Betula andPinus) was recorded, although various mesophilous and thermophilous tree elements were also present. The Younger Dryas, palynologically clearly defined, is characterized by an important reduction in tree pollen percentages and the expansion of steppe formations (Poaceae andArtemisia). At the beginning of the Holocene, there was an expansion ofQuercus and a spread of other trees, which combined to give a vegetation cover of varied composition but dominated by mixed deciduous forests. Such forest formations prevailed in these mountains until 3000 years ago, when successive deforestation phases are recorded at various times as a result of increased farming activity. The results are compared with data from other mountainous areas in the northern Iberian peninsula and southern France.     

Environmental changes recorded in deposits of the Izyubrinye Salontsi Lake,Sikhote-Alin

Paleoecological changes during the development of Lake Izyubrinye Solontsi from the Solontsovskie (Shanduyskie) Lakes located in the midlands of the Eastern Sikhote-Alin have been reconstructed. Lake formation is related to landslides on the paleovolcano slopes. A complex study of the peat-bog section (botanical, diatom, spore-pollen, and radiocarbon analysis) allows reconstructing paleoenvironmental changes with high resolution. A considerable variability of lake and swamp environments highly responsive to climate changes in the late Holocene has been revealed. Peat accumulation began about 4000¹⁴С BP. The main peat-forming plants were Sphagnum mosses and herbs, except for the period 2330?1530 ¹⁴C BP (2360?1480 cal. BP), when a swamp overgrown by larch forests and predominantly woody peat accumulated. Forest ecosystems on the lake coasts were quite stable. The role of fir and broadleaved species increased in the composition of dark coniferous forests with Korean pine during the warm phases and the role of birch, in the cold phases; secondary forests occupied the low slopes during the last 1000 years. The age of paleofires has been determined.     

A 37,500-year pollen record from Lake Malaŵi and implications for the biogeography of afromontane forests

Abstract. Two 10-metre cores were analysed and a 37,500 year pollen stratigraphy was established for Lake Mala?i. A lowstand from 37,500 to 35,900 BP indicates extremely dry conditions. From 35,900 to 34,000 BP montane forests were widespread indicating cold, moist climate. Between 34,000 and 26,400 BP warm and dry conditions prevailed and forest area decreased. During the Last Glacial Maximum montane forest was widespread in the catchment, although woodlands apparently persisted at low altitudes: the expansion of the montane forest indicates that the aridity that affected equatorial Africa during this time period did not affect the Lake Mala?i Catchment. The cold and relatively moist conditions during the Late Pleistocene probably allowed biotic interchange between the highlands of East and West Africa via the highlands along the Zaire—Zambezi watershed and among now-isolated islands of Afromontane vegetation in the Lake Mala?i catchment. The Holocene is characterized by climate and vegetation quite similar to today, with indications of slightly wetter conditions between 6150 and 3000 bp and slightly drier conditions between 8000 and 6150 bp . The low percentages of montane forest pollen throughout the Holocene support the hypothesis that the montane grasslands of Mala?i are not recently anthropogenic.     

Vegetation history and lake-level changes from the Younger Dryas to the present in Eastern Pyrenees (France): pollen, plant macrofossils and lithostratigraphy from Lake Racou (2000 m a.s.l.)

A multi-proxy palaeoecological investigation including pollen, plant macrofossil, radiocarbon and sedimentological analyses, was performed on a small mountain lake in the Eastern Pyrenees. This has allowed the reconstruction of: (1) the vegetation history of the area based on five pollen diagrams and eight AMS¹⁴C dates and (2) the past lake-level changes, based on plant macrofossil, lithological and pollen analysis of two stratigraphical transects correlated by pollen analysis. The palaeolake may have appeared before the Younger Dryas; the lake-level was low and the vegetation dominated by cold steppic grasslands. The lake-level rose to its highest level during the Holocene in the Middle Atlantic (at ca. 5060±45 b.p.). Postglacial forests (Quercetum mixtum and Abieto-Fagetum) developed progressively in the lower part of the valley, while dense Pinus uncinata forests rapidly invaded the surroundings of the mire and remained the dominant local vegetation until present. The observed lowering of the lake levels during the Late Atlantic and the Subboreal (from 5060 ± B.P. to 3590±40 b.p.) was related to the overgrowth of the mire. The first obvious indications of anthropogenic disturbances of the vegetation are recorded at the Atlantic/Subboreal boundary as a reduction in the forest component, which has accelerated during the last two millennia.     

Late Holocene ecological history of Pinus pinaster forests in the Sierra de Gredos of central Spain

This article describes the patterns and processes of vegetation change and fire history in the Late Holocene (c. 2400 calendar year BP) palaeoecological sequence of Lanzahíta, Sierra de Gredos in central Spain, and provides the first Iberian pollen sequence undertaken within a monospecific Pinus pinaster woodland. These new data reassess not only the autochthonous nature of this pine species in the region and the Iberian Peninsula, but also the naturalness of well-developed cluster pine forests. Conflicts of palaeoecological evidence with phytosociological models of vegetation dynamics in the study region, and the relationships of P. pinaster with fire occurrence in Mediterranean-type ecosystems, are discussed.     

云贵高原抚仙湖近13300年的花粉/炭屑记录*

抚仙湖是云贵高原著名的断陷深水湖,其沉积物蕴藏着流域地质历史时期丰富的环境信息。对钻取自该湖的900cm 湖泊沉积物岩芯进行花粉/炭屑分析及花粉数据的主成分分析表明,抚仙湖流域的植被、气候与火灾在过去的13 300年经历了5个阶段的变化：(1)13 300—10 400cal．a BP,植被以松林为主,伴有山地暗针叶林和常绿阔叶林,表明该时期气候较为冷湿,森林火灾多发,在后期随着温度和湿度的降低,森林火灾愈加频繁。(2)10 400—5 700cal．a BP,松林收缩,常绿阔叶林扩张,出现一定数量的落叶阔叶林,显示该时期气候偏暖偏干;此阶段早期随着气候变暖变干森林火灾的发生延续上阶段高发的状态,直到9 500cal．a BP后随着湿度的增加森林火灾明显减少。(3)5 700—1 800cal．a BP,松林变化较小,常绿/落叶阔叶林比重增大,首次出现了暖热性的枫香林,显示该时期暖湿的气候特征,火灾发生频率低。(4)1 800—500cal．a BP,松林扩张,阔叶林收缩,本阶段后期草本植被比重开始增加,显示该时期气候相对冷干,森林火灾发生频率较高。(5)500cal．a BP至今,松林收缩,落叶阔叶树种增多,草本植物花粉明显增多,显示该时期气候温凉偏干,森林火灾发生频率降低。     

Environmental Kuznets Curve hypothesis and the role of globalization in selected African countries

The present study incorporates globalization and energy intensity into the CO₂ emissions function and investigates the presence of Environmental Kuznets Curve (EKC) in 19 African countries for the time period of 1971–2012. We have applied the ARDL bounds testing approach for cointegration to examine the long run relationship in the variables. Our results confirmed the presence of cointegration between the series in Africa, Algeria, Angola, Cameroon, Congo Republic, Ghana, Kenya, Libya, Morocco, Nigeria, South Africa, Sudan, Tanzania, Togo, Tunisia, Zambia and Zimbabwe. The results indicated the positive effect of energy intensity on CO₂ emissions in Africa, Algeria, Angola, Cameroon, Congo Republic, Ghana, Kenya, Libya, Morocco, Nigeria, South Africa, Sudan, Togo, and Tunisia while energy intensity declines CO₂ emissions in the case of Zambia and Zimbabwe. Globalization decreases CO₂ emissions in Africa, Angola, Cameroon, Congo Republic, Egypt, Kenya, Libya, Tunisia and Zambia but increases CO₂ emissions in Ghana, Morocco, South Africa, Sudan and Tanzania. The EKC exists in Africa, Algeria, Cameroon, Congo Republic, Morocco, Tunisia and Zambia but U-shaped relationship is found between economic growth and CO₂ emissions in Sudan and Tanzania.     

Impact of a drier Early-Mid-Holocene climate upon Amazonian forests

This paper uses a palaeoecological approach to examine the impact of drier climatic conditions of the Early-Mid-Holocene (ca 8000-4000 years ago) upon Amazonia's forests and their fire regimes. Palaeovegetation (pollen data) and palaeofire (charcoal) records are synthesized from 20 sites within the present tropical forest biome, and the underlying causes of any emergent patterns or changes are explored by reference to independent palaeoclimate data and present-day patterns of precipitation, forest cover and fire activity across Amazonia. During the Early-Mid-Holocene, Andean cloud forest taxa were replaced by lowland tree taxa as the cloud base rose while lowland ecotonal areas, which are presently covered by evergreen rainforest, were instead dominated by savannahs and/or semi-deciduous dry forests. Elsewhere in the Amazon Basin there is considerable spatial and temporal variation in patterns of vegetation disturbance and fire, which probably reflects the complex heterogeneous patterns in precipitation and seasonality across the basin, and the interactions between climate change, drought- and fire susceptibility of the forests, and Palaeo-Indian land use. Our analysis shows that the forest biome in most parts of Amazonia appears to have been remarkably resilient to climatic conditions significantly drier than those of today, despite widespread evidence of forest burning. Only in ecotonal areas is there evidence of biome replacement in the Holocene. From this palaeoecological perspective, we argue against the Amazon forest 'dieback' scenario simulated for the future.     

Colombian dry moist forest transitions in the Llanos Orientales—A comparison of model and pollen-based biome reconstructions

Colombian vegetation, at the ecological level of the biome, is reconstructed at six sites using pollen data assigned a priori to plant functional types and biomes. The chosen sites incorporate four savanna sites (Laguna Sardinas, Laguna Angel, El Piñal and Laguna Carimagua), a site on the transition between savanna and Amazon rainforest (Loma Linda) and a site within the Amazon rainforest (Pantano de Monica). The areal extent of tropical moist forest, tropical dry forest and steppe have been subject to significant change: differential responses of the vegetation to climatic shifts are related to changes in plant available moisture, duration of dry season and edaphic controls on the vegetation. The record from El Piñal shows that the present-day savanna vegetation, dominated by steppe (Poaceae) with little occurrence of woody savanna taxa (e.g. Curatella, Byrsonima), was present since the last glacial period of the northern hemisphere. Unfortunately, El Piñal is located on an edaphic savanna and is not particularly responsive to registering change. Most records cover the early Holocene; one site records the El Abra stadial (Younger Dryas equivalent), when forest expansion reflects more humid climatic conditions and higher plant available moisture. During the early and middle Holocene, the maximum expansion of steppe and tropical dry forest occurred, indicating that dry climatic conditions continued to around 4000 ¹⁴C BP. The following period, from shortly before 4000 ¹⁴C BP, is characterised by an increase in forest and gallery forests, reflecting a wetter period probably with a shorter annual dry season. Anthropogenic influence on the vegetation is recorded by all the records over the last millennial, particularly characterised by a reduction in forest cover and high amplitude changes in vegetation.Biome transitions from one type to another, and the environmental controls on this shift, are investigated by applying a vegetation model (BIOME-3). The model uses climatic data from six meteorological stations that, encompass a range of environments within lowland Colombia, which are similar to the pollen data. The signals of vegetation change can be translated to the main environmental controls of temperature and moisture to indicate the degree of change needed in these parameters to record the vegetation change depicted by the pollen data. Moisture balance is the dominant control on driving vegetation change whether under seasonal or annual control. The combined reconstruction from pollen data and model output of biome-scale vegetation dynamics for lowland Colombia allows an understanding of the environmental controls to be developed.     

Holocene development of two calcareous spring fens at the Carpathian-Pannonian interface controlled by climate and human impact

There is still not enough palaeoecological data from the southwestern part of the Western Carpathians, where mountain ridges steeply rise from the dry and warm Pannonian basin. The reason is a low availability of sites with sediments harbouring fossil remains. In the Pova?ský Inovec Mts, two small protected calcareous wetlands occur in different geographical position and contain suitable sediments. One represents a foothill site (initiated ca 13,000 cal. BP) whereas the other a low-mountain site (initiated ca 7,400 cal. BP). We investigated fossil pollen, spores, and macroscopic remains of plants and molluscs from their sediments. We further reviewed archaeological data, constructed a macrophysical climate model (MCM) and confronted it with other palaeoclimatic proxies. Temperate deciduous trees (Quercus, Corylus and Ulmus) occurred since the Allerød, but their expansion was blocked by a harsh climate in Younger Dryas, when Larix, Pinus and Betula nana still occurred. The climate firstly moistened at ca 9,500 cal. BP and more distinctly at ca 8,500 cal. BP, which was reflected by a strong calcium carbonate precipitation and expansion of Tilia cordata t., Hedera helix, and Ustulina. Although the MCM predicted a rather stable climate since 8,000 cal. BP, certain changes in aquatic mollusc abundances may indicate hydrological fluctuations, as they are paralleled by changes in climate humidity indicated by other evidence from the Western Carpathians. Younger hydrological fluctuations may be alternatively explained by human activities as they correspond with macro-charcoal abundance and indicators of wetland openness. During their existence, both fens harboured only few fen plant and mollusc species specialized to low-productive sedge-moss fens. In the Middle Holocene both sites were encroached by woody plants (Alnus, Picea and Salix), as most other spring fens in the Western Carpathians. Contrary to some other spring fens with similar site conditions in the Western Carpathians, few fen specialists established in the study sites since deforestation, presumably because of severe disturbances caused by grazing and/or hemp retting instead of the usual mowing.     

A palaeoecological investigation into the role of fire and human activity in the development of montane grasslands in East Africa

Human activity has been widely implicated in the origin and expansion of montane grasslands in East Africa, yet little palaeoecological evidence exists to test whether these grasslands are natural or secondary. Pollen and charcoal data derived from two Holocene records in the Eastern Arc mountains of Tanzania are used as a case study to investigate the supposed secondary nature of montane grasslands in Africa. Fossil pollen data are used to detect vegetation change, and charcoal analysis is used to reconstruct fire history. The pollen data are characterised by stable proportions of local taxa suggesting permanence of grasslands throughout the past ~13,000 years. Recent increases in fire adapted taxa such as Morella point towards the development of a grassland/forest patch mosaic possibly associated with burning. However, robust evidence of human activity is absent from the records, which may be attributed to the late human occupation of the mountains. The records indicate long-term persistence of grasslands which, coupled with a lack of evidence of human activity, suggests that these grasslands are not secondary. These data support the hypothesis that grasslands are an ancient and primary component of montane vegetation in Africa, but that they experienced some expansion during the late Holocene as a result of changing fire regime.     

The roles of rivers and Pleistocene refugia in shaping genetic diversity in Praomys misonnei in tropical Africa

Aim This study aims to elucidate the phylogeography of the murid rodent Praomys misonnei and to document whether or not rain forest refugia and rivers structure patterns of diversity within this species. Location Tropical Africa, from Ghana to Kenya. Methods Patterns of genetic structure and signatures of population history (cytochrome b gene) were assessed in a survey of 229 individuals from 54 localities. Using maximum likelihood, Bayesian, network and genetic structure analyses, we inferred intra‐specific relationships and tested hypotheses for historical patterns of gene flow within P. misonnei. Results Our phylogenetic analyses reveal a strong phylogeographical structure. We identified four major geographical clades within P. misonnei: one clade in Ghana and Benin, a Nigerian clade, a West Central African clade and a Central and East African clade. Several subclades were identified within these four major clades. A signal of population expansion was detected in most clades or subclades. Coalescence within all of the major clades of P. misonnei occurred during the Middle Pleistocene and/or the beginning of Late Pleistocene. Main conclusions Our results suggest a role for both Pleistocene refugia and rivers in structuring genetic diversity in P. misonnei. This forest‐dwelling rodent may have been isolated in a number of forest fragments during arid periods and expanded its range during wetter periods. Potential forest refugia may have been localized in Benin–Ghana, south‐western Cameroon, southern Gabon, northern Gabon and eastern Democratic Republic of Congo–western Uganda. The Niger and/or the Cross Rivers, the Oubangui‐Congo, Sanaga, Ogooue and/or Ivindo Rivers probably stopped the re‐expansion of the species from relict areas.     

Three new species of Stiphrornis (Aves: Muscicapidae) from the Afro-tropics,with a molecular phylogenetic assessment of the genus

We describe three new species of forest robin in the genus Stiphrornis; two from West Africa and one from the Congo Basin. Each species represents a distinct phylogenetic lineage based on genetic analysis. In addition to genetic differentiation, each new species is diagnosable from other Stiphrornis lineages by morphology, and by plumage. One of the new species appears to be restricted to the Central and Brong-Ahafo Regions of Ghana, and another is restricted to Benin and the Central Region of Ghana. In Ghana, these two new species presumably come into contact with Stiphrornis erythrothorax (Western Region of Ghana and westward), and there is evidence that one of the new species has a distinguishably different song from erythrothorax. The distribution of the third new species is primarily on the south bank of the Congo River, near the city of Kisangani. Recognition of these species provides additional evidence that Afrotropical forests are harbouring substantial cryptic diversity, and that our knowledge of the drivers of this diversity remains poorly documented across the region.

http://www.zoobank.org/urn:lsid:zoobank.org:pub:BF2A0BE6-1140-4EFF-9035-380D61AB03AE     

Holocene Palynological Records in Lake Selincuo,Northern Xizang

Selincuo (31°34′–31°57′N, 88°31′–89°21′E)is one of the great lakes in northern Xizang surrounded by alpine grassland, composed mainly of Stipa purpurea, S, subssessifolia var. bassipulomosa. A 3.08 m long core has been taken from a water depth of 27 m, 3 km to the north from the lake. Palynological records of the core combined with surface sample results have revealed from the regional as well as local vegetational and environmental history since the last 12 000 a BP, a very low pollen concentration (19–209 grains/g), mainly of airborne pollen, which indicated a treeless alpine sparse vegetation. A great vegetation change took place at Ca. 9 600 a BP when alpine sparse vegetation was replaced by alpine grassland lasting until Ca. 6 000 a BP. Quite a number of tree pollen grains found in this time interval might be explained as a result of expansion of forests somewhere around the Xizang Plateau. These changes must have caused by climatic warming during the Early Holocene. During the last 6 000 years the vegetation had remained as the alpine grassland, although probably more luxurouste than before, but their was an obvious increase of arboreal pollen, such as the increase of Abies during Ca. 6 000–4 000 a BP, Picea 4 000–2 200 a BP, Pinus 2 200–1 000 a BP which indicated continuous expansion of forests growing around the plateaus. During the last 1 000 years both the concentration and proportion of the arboreal pollen decreased and those of Cypraceae pollen increased sharply. This might mean a reduction of the forests around the plateau and an expansion of swamps around the lake.     

An 11?000-year lithostratigraphic and paleohydrologic record from Equatorial Africa: Lake Edward, Uganda-Congo

Sedimentologic and geochemical analyses of four cores from Lake Edward, Uganda-Congo, document a complex record of moisture balance in the Edward basin from 11?000 cal yr BP to present. Highly organic, diatomaceous muds provide evidence for shifts in wind intensity and stratification within an early Holocene wet phase. Lake level variations within this period may have shifted due to tectonic lowering of the lake’s outlet level. The onset of mid-Holocene aridity, as documented by the appearance of authigenic calcite at 5200 cal yr BP, initiated a period of falling lake levels that culminated in a late Holocene lowstand between 4000 and 2000 cal yr BP. This lowstand is documented by coarse sediments whose fabric and mineralogy depend on the core site. Although different cores yield different ages for this regression, it appears that declining lake levels culminated in a maximum lake lowstand of −14 m. Lake levels then rose rapidly, attaining modern positions by 1700 cal yr BP. This lake level history suggests that although many paleoclimatic changes in Africa are apparently synchronous throughout northern hemispheric Africa, other events may be spatially heterogeneous. These patterns highlight the need for a well-dated network of paleoclimate sites within the African continent.