Lake Barombi Mbo Research Articles

Trophic specialization on unique resources despite limited niche divergence in a celebrated example of sympatric speciation.

Trophic niche partitioning is observed in many adaptive radiations and is hypothesized to be a central process underlying species divergence. However, patterns of dietary niche partitioning are inconsistent across radiations and there are few studies of niche partitioning in putative examples of sympatric speciation. Here, we conducted the first quantitative study of dietary niche partitioning using stomach contents and stable isotope analyses in one of the most celebrated examples of sympatric speciation: the cichlid radiation from crater lake Barombi Mbo, Cameroon. We found little evidence for trophic niche partitioning among cichlids, including the nine species coexisting in the narrow littoral zone. Stable isotope analyses supported these conclusions of substantial dietary overlap. Our data, however, did reveal that five of eleven species consume rare dietary items, including freshwater sponge, terrestrial ants, and nocturnal foraging on shrimp. Stomach contents of the spongivore (Pungu maclareni) were 20% freshwater sponge, notable considering that only 0.04% of all fishes consume sponges. Overall, we conclude that cichlid species in lake Barombi Mbo overlap considerably in broad dietary niches-in part due to the large proportion of detritus in the stomach contents of all species-but there is evidence for divergence among species in their diet specializations on unique resources. We speculate that these species may utilize these additional specialized resources during periods of low resource abundance in support of Liem's paradox.

Description of a new species from Clarias maclareni and phylogenetical analysis of Quadriacanthus (Monogenea, Dactylogyridae) species transfers between clariid and non-clariid fish hosts in Cameroon.

Recently in Cameroon, two species belonging to Quadriacanthus: Q. anaspidoglanii Akoumba, Tombi & Bilong Bilong, 2017 and Q. euzeti Nack, Pariselle & Bilong Bilong, 2016 have been recorded on gill filaments of Notoglanidium macrostoma (Siluriformes, Claroteidae) in the Memou’ou River (Nyong Basin) and Papyrocranus afer (Osteoglossiformes, Notopteridae) in Lake Ossa, respectively. These records have been considered the result of lateral transfers from Clariidae to a Claroteidae host for the first case (parasitism of N. macrostoma by Q. anaspidoglanii) and from Clariidae or Bagridae to a Notopteridae host for the second (parasitism of P. afer by Q. euzeti). In this paper, the investigation of interspecific relationships among Quadriacanthus spp. parasitizing Clariidae, Bagridae, Claroteidae and Notopteridae in Cameroon resulted in the record of Q. anaspidoglanii from N. macrostoma, Q. euzeti from P. afer, a new record of Q. levequei Birgi, 1988 from Clarias jaensis in the Nyong River, and the description of Q. barombiensis n. sp. from Clarias maclareni in Lake Barombi Mbo. The newly identified species is characterized by having an accessory piece ending in one small hook and the median expansion of its dorsal bar with two filaments. Phylogenetic analysis based on 28S rDNA sequences confirms that the Quadriacanthus spp. parasitizing gill filaments of non-clariid hosts in Cameroon originate from lateral transfers from clariid fishes, and that Clariidae are ancestral hosts of these monogenean species.

Understanding the 2500 yr BP rainforest crisis in West and Central Africa in the framework of the Late Holocene: Pluridisciplinary analysis and multi-archive reconstruction

Abstract Numerous palaeoclimatic and prehistoric reconstructions have been reported at both the local scale and across the very large area of tropical Central Africa, clearly highlighting the Late Holocene Rainforest Crisis (LHRC) which mainly developed from ca. 2500 to 2000 yr BP. The broad synchroneity of this interval is striking and has been revealed in many different deep lakes from humid or montane forest, lakes or swamps marginal to tropical forest, and swamp forests where the oscillations of the surface water table produce temporary emersions. In parallel, a chronological review is presented of the Bantu arrival in these areas, including indicators for burning, metallurgy and land clearance. Nevertheless, evidence of human occupation, such as artefacts or deposits containing charcoal is exceptional and generally absent in most areas. Although the archaeological data exhibit a gradual southward densification of human occupation throughout Central Africa, the increase of settlements clearly began after 2350 yr BP, not before 2500 BP. However, some authors have interpreted the geochemical signal of increased erosion in the Congo Basin or the opening up of the forest around Lake Barombi Mbo in Cameroon as being attributed to agricultural clearance, or even the supply of charcoal required for metallurgy. In short, these early Bantu settlers (in such modest densities) may have been responsible for some local landscape degradation (clearance, fire, metallurgy), but these same settlers could not, under any circumstances and across all Central Africa, be held responsible directly or indirectly for the synchronous changes of lake levels, draining vast swamps and opening up of the tropical forest canopy, which was due to an increased dry season, while the recovery with the recolonization of light demanding species, including oil palms (Elaeis guineensis) assisted by dispersers such as chimpanzees, was likewise due to natural processes. Therefore, it can be concluded that no data available validate the hypothesis that the major erosion or vegetation destruction ca. 2500 yr BP, was the result of large population movements. Indeed, the evolution of the environments of Central Africa are linked to the natural responses induced by general palaeoclimatic processes, observed synchronously not only in northern and eastern Africa, but globally. The natural recovery and resilience of these forests until the last centuries contrasts with the situation currently being faced.

Major/trace elements and Sr–Nd–Pb isotope systematics of lavas from lakes Barombi Mbo and Barombi Koto in the Kumba graben, Cameroon volcanic line: Constraints on petrogenesis

This study presents major elements, trace elements and Sr–Nd–Pb isotope data for fifteen (15) representative volcanic rock samples from volcanic edifices of Lakes Barombi Mbo (n = 9) and Barombi Koto (n = 6), located in the Kumba graben along the Cameroon Volcanic Line (CVL). The samples are dominantly basalts and trachy-basalts with MgO ranging from 7 to 9 wt.%. The lavas from the edifice of Lake Barombi Mbo are slightly richer in TiO2 (mean 3.3 wt%) and lower in CaO (mean 9.2 wt%) relative to those from the edifice of Lake Barombi Koto (2.9 wt% and 10.3 wt%, respectively). Trace element characteristics of the lavas suggest an enrichment of the source by carbonatitic metasomatism and melting of a heterogeneous source at varying depth in the garnet-spinel transition zone. Trace elements and isotopic composition of the lavas suggest insignificant crustal contamination. Modelling of the degree of partial melting suggest that the magmas were formed at varying depth (60–80 km) and by ~2% partial melting of a source containing less than 4% garnet. Samples from Lake Barombi Mbo formed at a higher degree of partial melting (2%–0.5%) and at a shallower depth relative to those from the volcanic edifice of Lake Barombi Koto (mostly <0.5%) and Mt. Cameroon. Interestingly, the relationship of Zr/Hf and 206Pb/204Pb suggests that the magma was affected by carbonatitic metasomatism whose effect decreased from Mt. Cameroon to Lake Barombi Mbo. Our results suggest that Lakes Barombi Mbo and Barombi Koto lavas are typical of lavas with HIMU-like signature and involved contributions from a dominantly sublithospheric mantle (SLM) and minor contributions from the sub-continental lithospheric mantle (SCLM). The SLM material rose and was hosted in the SCLM prior to melting.

Evolution of the visual sensory system in cichlid fishes from crater lake Barombi Mbo in Cameroon.

In deep-water animals, the visual sensory system is often challenged by the dim-light environment. Here, we focus on the molecular mechanisms involved in rapid deep-water adaptations. We examined visual system evolution in a small-scale yet phenotypically and ecologically diverse adaptive radiation, the species flock of cichlid fishes in deep crater lake Barombi Mbo in Cameroon, West Africa. We show that rapid adaptations of the visual system to the novel deep-water habitat primarily occurred at the level of gene expression changes rather than through nucleotide mutations, which is compatible with the young age of the radiation. Based on retinal bulk RNA sequencing of all eleven species, we found that the opsin gene expression pattern was substantially different for the deep-water species. The nine shallow-water species feature an opsin palette dominated by the red-sensitive (LWS) opsin, whereas the two unrelated deep-water species lack expression of LWS and the violet-sensitive (SWS2B) opsin, thereby shifting the cone sensitivity to the centre of the light spectrum. Deep-water species further predominantly express the green-sensitive RH2Aα over RH2Aβ. We identified one amino acid substitution in the RH2Aα opsin specific to the deep-water species. We finally performed a comparative gene expression analysis in retinal tissue of deep- vs. shallow-water species. We thus identified 46 differentially expressed genes, many of which are associated with functions in vision, hypoxia management or circadian clock regulation, with some of them being associated with human eye diseases.

Molecular basis of adaptation to hypoxia in Barombi Mbo crater lake cichlids

Event Abstract Back to Event Molecular basis of adaptation to hypoxia in Barombi Mbo crater lake cichlids Dmytro Omelchenko1*, Pavel Talacko1, Adrian Indermaur2, Walter Salzburger2 and Zuzana Musilova1 1 Charles University, Czechia 2 University of Basel, Switzerland Deep water zone represents an extreme environment where organisms have to cope with low dissolved oxygen concentrations, low-intensity light with the narrower color spectrum and high hydrostatic pressure. Such conditions create selective pressure and shape the evolution of living organisms in deep waters. They establish various adaptations to exploit energy sources which are unavailable for shallow-water inhabitants. The general challenge of adaptation research is to identify the link between the species ecology, environment and the molecular adaptation in different species. Here we study endemic cichlid fishes from the crater lake Barombi Mbo in Cameroon, West Africa. We compare shallow-water and deep-water cichlid species from monophyletic species flock focusing on gene sequence, expression levels and proteome abundance of hemoglobins and other candidate proteins (myoglobin, coagulation factors) that might be involved in establishment of adaptation to life in the depth. We performed genome assembly of 4 Barombi Mbo cichlid species and obtained information about exact synteny of hemoglobin gene clusters. We inferred CDS structure of hemoglobin genes with subsequent phylogenetic analysis. In addition, we inspected the hemoglobin clusters in the closely related species, Nile tilapia, and identified 17 Hb-alpha and 15 Hb-beta genes. According to our knowledge Nile Tilapia has the highest number of hemoglobin genes among Vertebrates. Furthermore, we performed RNA sequencing of the gill tissue and calculated gene expression for hemoglobin subunits. We estimated that from multiple hemoglobin genes (18 in Barombi Mbo cichlids) only four are expressed in adult fish. We observed that those are two pairs of alpha and beta subunits which are located in the close synteny. We further estimated that expression in deep-water species is slightly higher and favored towards one pair of the hemoglobin genes while in shallow-water species expression is shifted towards another pair of hemoglobin subunit genes. This might be connected to our observation that allele of the genes (Hb-beta 11 and Hb-alpha 12) that have higher expression in two deep-water species possessing 4 amino acid substitutions (3 in Hb-beta 11 and 1 in Hb-alpha 12 respectively). Moreover, we performed targeted and non-targeted HPLC-MS/MS analysis of blood proteome and identified 23 proteins that are differentially abundant in fishes coming from different depth horizons. We demonstrate that adaptation to hypoxia in Barombi Mbo crater lake cichlids is likely reflected on several levels including structural changes within coding region of hemoglobin genes and their expression. In addition, we provide evidence of engagement of various proteins in hypoxia adaptation. Keywords: Cichlids, Hemoglobin, hypoxia, Proteome, adaptation, Differential gene expression, crater lake Conference: XVI European Congress of Ichthyology, Lausanne, Switzerland, 2 Sep - 6 Sep, 2019. Presentation Type: Oral Topic: GENETICS, GENOMICS AND PHYSIOLOGY Citation: Omelchenko D, Talacko P, Indermaur A, Salzburger W and Musilova Z (2019). Molecular basis of adaptation to hypoxia in Barombi Mbo crater lake cichlids. Front. Mar. Sci. Conference Abstract: XVI European Congress of Ichthyology. doi: 10.3389/conf.fmars.2019.07.00062 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 21 Jul 2019; Published Online: 14 Aug 2019. * Correspondence: Mr. Dmytro Omelchenko, Charles University, Prague, Czechia, omeldima@gmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Dmytro Omelchenko Pavel Talacko Adrian Indermaur Walter Salzburger Zuzana Musilova Google Dmytro Omelchenko Pavel Talacko Adrian Indermaur Walter Salzburger Zuzana Musilova Google Scholar Dmytro Omelchenko Pavel Talacko Adrian Indermaur Walter Salzburger Zuzana Musilova PubMed Dmytro Omelchenko Pavel Talacko Adrian Indermaur Walter Salzburger Zuzana Musilova Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Urban Development and its Implications on Wetland Ecosystem Services in Ndop, Cameroon

Wetlands provide a diversity of ecosystem services (provisioning, regulatory, supporting and cultural services) which are essential for the human society. This is particularly necessary in cases where large urban agglomerations exist. These services are increasingly coming under pressure as a result of rapid and uncontrolled urban development which remains phenomenal in the developing world. Cameroon can boast of a number of wetland environments particularly, the Waza Logone flood plain, the wetlands of the Bakassi Peninsula, Lake Barombi Mbo and the Ndop Plains. The wetland ecosystems of the Ndop Plains have increasingly come under pressure as a result of urban development which is mirrored in population growth, housing and other infrastructural developments. While significant research works have been undertaken on aspects related to wetlands, agriculture and climate variability in the Ndop plains, there is insufficiency scientific information on the implications of urban development on wetland ecosystem services. Using questionnaire (where we systematically sample 140 households) and Landsat images which were complemented by field observations, relevant data were generated. With the aid of SPSS (version 21), the correlation between wetland ecosystem services and urban development at 0.05 level of significance was analysed. Our results reveal a significant negative relationship (r = -0.551 and -0.682) between urban development and the state of wetlands and their ecosystem services. This was further buttressed by geospatial data which revealed that the Ndop urban space increased from 3.7km2 in 1999 to 11.7km2 in 2017. In this regard, we suggest that the coordination of the urban development process through land use planning and zoning is imperative in the face of unabated urban development. We equally suggest the need to implement wetland management policies in line with the Ramsar convention’s paradigm on the “wise use” of wetlands.

Determination of water quality, and trace metals in endemic Sarotherodon linellii, Pungu maclareni and Clarias maclareni, in crater Lake Barombi Mbo, Cameroon

Considering water pollution as a potential threat to some endemic cichlids of Lake Barombi Mbo, Cameroon, an investigation was done in 2011 to determine trace metals in its water, linking their uptake in gills and liver of fish to water chemistry. ICP-MS and ICP-OES analyses of trace metals based on total concentration of unfiltered lake water samples showed the presence of trace metals. All fish species accumulated Al, Mn and Sr in the highest concentrations in their gills, with Cu, Cd, Co, Cr, Pb and U highest in the liver. Pungu maclareni accumulated Al, Cr, Co, Sr and Pb in the highest concentrations. The highest mean gill Al concentration of 140 µg g−1 dry weight was measured in P. maclareni gills, this being one of the critically endangered cichlids of the lake. Stable isotope analyses of carbon δ13C and nitrogen δ15N showed that P. maclareni had the highest mean δ13C (−30.2‰) and highest concentrations of Cr, Co, Pb and U in liver, probably linking the carbon source to the accumulation of metals. Though trace metal levels in the lake water were low, their presence in fish tissues suggest they are bioavailable, bioaccumulate and may pose a threat to the aquatic biota, and therefore should be monitored.

Threats to Ecotourism Development and Forest Conservation in the Lake Barombi Mbo Area (LBMA) of Cameroon

There is increasing recognition of the fact that the development of ecotourism could be very instrumental in the drive towards sustainable development, since it involves the improvement of liveliho...

Genetic signatures of a demographic collapse in a large‐bodied forest dwelling primate (Mandrillus leucophaeus)

It is difficult to predict how current climate change will affect wildlife species adapted to a tropical rainforest environment. Understanding how population dynamics fluctuated in such species throughout periods of past climatic change can provide insight into this issue. The drill (Mandrillus leucophaeus) is a large-bodied rainforest adapted mammal found in West Central Africa. In the middle of this endangered monkey's geographic range is Lake Barombi Mbo, which has a well-documented palynological record of environmental change that dates to the Late Pleistocene. We used a Bayesian coalescent-based framework to analyze 2,076 base pairs of mitochondrial DNA across wild drill populations to infer past changes in female effective population size since the Late Pleistocene. Our results suggest that the drill underwent a nearly 15-fold demographic collapse in female effective population size that was most prominent during the Mid Holocene (approximately 3-5 Ka). This time period coincides with a period of increased dryness and seasonality across Africa and a dramatic reduction in forest coverage at Lake Barombi Mbo. We believe that these changes in climate and forest coverage were the driving forces behind the drill population decline. Furthermore, the warm temperatures and increased aridity of the Mid Holocene are potentially analogous to current and future conditions faced by many tropical rainforest communities. In order to prevent future declines in population size in rainforest-adapted species such as the drill, large tracts of forest should be protected to both preserve habitat and prevent forest loss through aridification.

Pollen, vegetation change and climate at Lake Barombi Mbo (Cameroon) during the last ca. 33 000 cal yr BP: a numerical approach

Abstract. This paper presents quantitative reconstructions of vegetation and climate along the pollen sequence of Lake Barombi Mbo, southwestern Cameroon (4°39'45.75" N, 9°23'51.63" E, 303 m a.s.l.) during the last 33 000 cal yr BP, improving previous empirical interpretations. The biomisation method was applied to reconstruct potential biomes and forest successional stages. Mean annual precipitation, mean annual potential evapotranspiration and an index of moisture availability were reconstructed using modern analogues and an artificial neural network technique. The results show a dense forested environment around Lake Barombi Mbo of mixed evergreen/semi-deciduous type during the most humid phases (highest precipitation and lowest evapotranspiration), but with a more pronounced semi-deciduous type from ca. 6500 cal yr BP to the present day, related to increased seasonality. This forest displays a mature character until ca. 2800 cal yr BP, then becomes of secondary type during the last millennium, probably due to increased human activity. Two episodes of forest fragmentation are shown, which are synchronous with the lowest reconstructed precipitation and highest potential evapotranspiration values. The first of these occurs during the LGM, and the second one from ca. 3000 to ca. 1200 cal yr BP, mainly linked to high precipitation seasonality. Savanna were, however, never extensive within the Barombi Mbo basin, existing instead inside the forest in form of savanna patches. The climate reconstructions at Lake Barombi Mbo suggest that the artificial neural networks technique would be more reliable in this region, although the annual precipitation values are likely under-estimated through the whole sequence.

Reticulate sympatric speciation in Cameroonian crater lake cichlids

BackgroundTraditionally the rapid origin of megadiverse species flocks of extremely closely related species is explained by the combinatory action of three factors: Disruptive natural selection, disruptive sexual selection and partial isolation by distance. However, recent empirical data and theoretical advances suggest that the diversity of complex species assemblages is based at least partially on the hybridization of numerous ancestral allopatric lineages that formed hybrids upon invasion of new environments. That reticulate speciation within species flocks may occur under sympatric conditions after the primary formation of species has been proposed but not been tested critically.ResultsWe reconstructed the phylogeny of a complex cichlid species flock confined to the tiny Cameroonian crater lake Barombi Mbo using both mitochondrial and nuclear (AFLP) data. The nuclear phylogeny confirms previous findings which suggested the monophyly and sympatric origin of the flock. However, discordant intra-flock phylogenies reconstructed from mitochondrial and nuclear data suggest strongly that secondary hybridization among lineages that primarily diverged under sympatric conditions had occurred. Using canonical phylogenetic ordination and tree-based tests we infer that hybridization of two ancient lineages resulted in the formation of a new and ecologically highly distinct species, Pungu maclareni.ConclusionsOur findings show that sympatric hybrid speciation is able to contribute significantly to the evolution of complex species assemblages even without the prior formation of hybrids derived from allopatrically differentiated lineages.

The dynamic of organic carbon in South Cameroon: fluxes in a tropical river system and a lake system as a varying sink on a glacial–interglacial time scale

In the first attempt to estimate both (i) a bulk carbon flux in a tropical river system (mainly Sanaga River) and (ii) their palaeoenvironmental implications from the Last Glacial Maximum (LGM) to the present, this study presents a synthetic approach based on the combined use of modern evaluation of fluxes and estuarine biodegradation in the tropical river system Sanaga and nearby Douala Bay rivers, and of sedimentation rates of a well studied marine shelf and lake system (Barombi-Mbo). In the lake Barombi-Mbo, the Holocene transfer of particulate carbon (96.6×10 3 t) is very close to the mass fixed presently in soil catchments (117×10 3 t). A complete process of stored carbon consumption would require some 10 4 years, namely the Holocene period. During the last 20,000 years, variations in the sediment organic matter can be explained by the change of the vegetation cover, particularly with the substitution of open environments by forests. The global sedimentation was slow between ca. 18,000 and 10,000 years BP and increased after 12,000 years. But the carbon sedimentation rate remains fairly constant as the carbon content is higher in the LGM deposits. Such LGM carbon concentrations are probably explained by the input of coarse debris by rough floods and by a less degraded organic matter as a result of the cooling of the climate. Today, the total transport of dissolved and particulate organic carbon of the Sanaga and Douala Bay rivers to the Guinea Gulf is estimated as 0.62 to 0.79×10 6 t C yr −1. Based on 50% biodegradation at the estuarine interface, the loss of organic matter per unit of land is evaluated around 8.8 t C km −2 yr −1. Marine oceanic records of the carbon sedimentation rate reflect with difficulty the major palaeoenvironmental changes according to interfering hydrodynamic factors. The greatest input of organic carbon during warm marine biozones would be balanced by higher concentrations during the LGM resulting in a nearly homogenous carbon transfer during the last 20,000 years. Such results might be largely representative of tropical river system as the contrasting vegetal cover (savanna and forest) of the Sanaga basin reflected as well the majority of the intertropical ecosystem. Thus, an estimate of the Holocene transfer to the ocean up to four times the present carbon stored in soil of the surrounding continent implicates that the Holocene shelf was a significant organic carbon sink. Although the sources of the Sanaga River are located in a mountain region, a significant floodplain is not found downstream. This results in a significant altitudinal factor in the carbon fluxes to the ocean.

Vegetation dynamics, palaeoenvironments and climatic changes in the forests of western Cameroon during the last 28,000 years B.P.

The lake Barombi Mbo pollen record goes back to about 28,000 yr B.P. The pollen diagram based on 82 samples is subdivided into four main pollen zones. Zone I (ca. 28,000 to 20,000 yr B.P.) is characterized by relatively high frequencies of Caesalpiniaceae and also by a montane element with Olea capensis. The climate was cool and relatively wet. Zone II (ca. 20,000 to 10,000 yr B.P.). A sharp increase in Gramineae, the main non-arboreal land pollen taxon, began around 20,000 yr B.P. and lasted until 10,000 yr B.P. During this period the forest receded, giving way to a more open vegetation, but significant patches of forest (refuges) persisted in the area. This is confirmed by isotopic analyses (δ 13C of sedimentary detritic organic matter from the catchment. Until ca. 13,000 yr B.P. Olea capensis was well represented indicating a relatively cool climate. Between 13,000 and 12,000 yr B.P. a warming trend associated with a strong increase in precipitation occurred. After this an abrupt reduction in precipitation linked to an increase in seasonality, but without temperature lowering, intervened between ca. 11,500 and 10,400 yr B.P. This last phase corresponds partly to the Younger Dryas time period. Zone III (ca. 10,000 to 2800 yr B.P.). After ca. 10,400 yr B.P. the climate became very wet until ca. 3000 yr B.P. A sharp decrease in the Gramineae intervened at ca. 10,000 yr B.P.; from ca. 9500 to 3000 yr B.P. they remained very low, between 0 and 3%, and the forest trees reached their maximum extension. Most of the trees exhibited large variations with quasi-periods of around 1000 to mainly 2000 yr (ca. 2200 calendar years), which could be related to large sylvigenetic or successional cycles. In this zone the Caesalpiniaceae were relatively well represented, with a maximum extension between 4500 and 3000 yr B.P. Podocarpus, a typical tree of the montane stratiform cloud forests, exhibited very low frequencies before 10,000 yr B.P. but their relative increase during the early and middle Holocene can only be explained by its growth on distant mountains. Its maximum extension phase was roughly synchronous with that of Caesalpiniaceae. The climate was warm and wet, but cooler on the mountains. Zone IV (ca. 2800 yr B.P. to present time). Around 2800 yr B.P. a sharp increase in the Gramineae, peaking at 30 to 40% of total pollen between ca. 2500 and 2000 yr B.P., indicates a sudden phase of vegetation opening and forest retreat, accompanied by severe erosion. Alchornea, a typical pioneer taxon, increased rapidly at the same time to large frequencies because it develops abundantly in all the openings. Elaeis guineensis, originally a pioneer palm tree, follows the same pattern. The climate was warm, relatively dry, and linked to an increase of seasonality. After 2000 yr B.P. the Gramineae returned to low frequencies, around 10%, associated with a strong increase in trees, indicating that the forest expanded again but not to the same extent as in the early and middle Holocene. The climate was warm and relatively wet, rather similar to the present-day climate.

Late Quaternary palaeoenvironments in the Lake Barombi Mbo (West Cameroon) deduced from pollen and carbon isotopes of organic matter

A sediment core, 23.5 m long, was recovered from a water depth of ca. 110 m in Lake Barombi Mbo, a maar crater of the Cameroon volcanic chain. This paper presents a carbon isotopic curve of organic matter linked to the main results of the pollen analysis. Interpretation of this carbon isotopic curve is simplified (1) because of the weak role played by the phytoplankton in the balance of the organic particle flux, and (2) because of the presumed absence of diagenetic alteration of the isotopic composition. The carbon isotopic curve exhibits an almost linear correlation with that of total grass pollen which form the main part of the C4 biomass. The major fluctuations of isotopic and pollen curves allow four main phases to be distinguished: • -from ca. 25,000 to 20,000 yr B.P., δ 13C values of −25 to −30‰ (PDB) are related to a forest environment associated with a montane floral element; • -from 20,000 to 13,000 yr B.P., the δ 13C values are between −23 and −28ℵ. and are linked to openings of landscape with a mosaic of forest and savanna and a fall of lake level with colonization of the shores by Cyperaceae and other aquatic plants; • -from 13,000 to 10,000 yr B.P., the forest extended again reaching a maximum density during the period from 9500 until 3000 yr B.P. and with an average δ 13C value of −32‰; • -a new dry phase at around 2500-2000 yr B.P., with temporary openings in the forest, marked by a δ 13C excursion above −30‰, but during the last 2 millennia, the forest developed again with δ 13C values of −32‰.

Sedimentation and palaeoenvironment in crater lake Barombi Mbo, Cameroon, during the last 25,000 years

Lake Barombi Mbo is situated in a maar crater of the Cameroon volcanic chain. Sediment cores were recovered with a Livingstone corer. This paper presents data on core BM6, 23.5 m long, from a water depth of 110 m. The sediments are mainly organic matter-rich clays (5 to 10% total organic carbon) showing alternating brown and yellowish horizontal laminae on a cm-scale. The lower part of a couplet (basal sublamina) is rich in siliciclastic (quartz, mica) grains, sponge spicules and documents the deposition of flood detritus. The upper unit (upper sublamina) is composed of grey to bluish clay which becomes green toward the top with frequently yellow, layered concretions of siderite. It corresponds to quiet conditions with settling of fine clayey particles from suspension. The overall recurrence rate of these laminae is about 15 years. This frequency is similar to present-day large flood events in this intertropical zone. From 14,000 to 20,000 yrs BP, the sediment accumulation rate was quite low (15–30 g cm −2 10 −3 yrs). With less abundant precipitation, pedogenic clay formed from mica-family minerals. From 12,000 yrs BP throughout the Holocene, the rate of accumulation increased (> 40 g cm −2 10 −3 yrs) and increasing precipitation favoured the pedogenic formation of kaolinites. Turbidity currents became less important to the overall sedimentation. Plankton production remained low, documented by organic matter which is more of the exinite than the alginite type. Siderite concretions occur as early diagenetic products, possibly related to alkalinity in anoxic waters at the sediment-water interface which limited the formation of Fe 3+. Thin ash layers occur between 11.5 and 18 m depth in core BM6, bearing witness to several volcanic events during the Late Quaternary. Sediments are disturbed below 21 m with folded laminations and inverse ages. These probably result from a slump glide of lower slope sediment but could also signal a possible gas event.

Chronological potential of palaeomagnetic oscillations recorded in late quaternary sediments from Lake Tanganyika

The measurements of Depositional and Post-Depositional Remanent Magnetizations (DRM and PDRM) in two piston cores collected at water depths of 140 m (core MPU3) and 422 m (core MPU12) in the southern Lake Tanganvika (Mpulungu Sub-Basin, Zambia) provide a palaeomagnetic record for south-east Central Africa, which spans over the last 25-5 ka. Ages (on a conventional 14C time scale) for the most characteristic inclination peaks were estimated by correlating the palaeomagnetic record with the Lake Barombi-Mbo (Cameroon) and the Western Europe Secular Variation type curves. AMS 14C ages on Total Organic Matter (TOM) measured in MPU12 core appear to be 2 to 4 ka older in the lower part of the record, from ca. 25 to 15 ka BP, i.e. during the last arid episode of the Late Pleistocene. These apparent 14C ages exceed PDRM acquisition lagtimes (evaluated to be about 800 years at most), and also account for residence time of carbon in TOM prior to its sedimentation. Therefore magnetostratigraphic correlations are believed to provide a better overall chronology for the Late Pleistocene and Early Holocene changes in Lake Tanganyika level. Major drops shown by breaks of deposition in core MPU3 (at 1000 cm and 825 cm sub-bottom) are respectively dated at ca. >21.5 ka BP and between 17 and 15 ka BP.

Lithostratigraphy, volcanism, paleomagnetism and palynology of Quaternary lacustrine deposits from Barombi Mbo (West Cameroon): Preliminary results

We present preliminary results from the study of 23.50-m core from Lake Barombi Mbo, representing the last 25,000 years. The lake is in an explosion crater formed during Quaternary time. The very laminated sediment is composed mostly of clay containing 5–10% organic carbon. Each couplet is commonly composed of a basal lamina rich in quartz, plant debris, muscovite and sponge spicules, and of a more clayey upper lamina often with siderite. A perturbed section near the base of the core, before ca. 21,000 yr B.P., could be the result of a violent release of gas, such CO 2 , comparable to the recent Nyos gas eruption. The paleomagnetic studies exhibit high-frequency oscillations interpreted as paleosecular variations of the local geomagnetic field. This first record obtained on the African continent can be closely compared to the type record obtained in Western Europe. The pollen results demonstrate the presence of a forest refuge in West Cameroon during the last great arid period, ca. 18,000 yr B.P. When equatorial forest was broken up, elements of montane vegetation spread to the lowlands. These phenomena resulted from a drying and cooling climate.

Lake Bambuluwé (Cameroon): Building-up the same scenario as Lake Nyos?

We present preliminary results from the study of 23.50-m core from Lake Barombi Mbo, representing the last 25,000 years. The lake is in an explosion crater formed during Quaternary time. The very laminated sediment is composed mostly of clay containing 5–10% organic carbon. Each couplet is commonly composed of a basal lamina rich in quartz, plant debris, muscovite and sponge spicules, and of a more clayey upper lamina often with siderite. A perturbed section near the base of the core, before ca. 21,000 yr B.P., could be the result of a violent release of gas, such CO2, comparable to the recent Nyos gas eruption.The paleomagnetic studies exhibit high-frequency oscillations interpreted as paleosecular variations of the local geomagnetic field. This first record obtained on the African continent can be closely compared to the type record obtained in Western Europe.The pollen results demonstrate the presence of a forest refuge in West Cameroon during the last great arid period, ca. 18,000 yr B.P. When equatorial forest was broken up, elements of montane vegetation spread to the lowlands. These phenomena resulted from a drying and cooling climate.

Palaeomagnetic study of the Holocene and Upper Pleistocene sediments from Lake Barombi Mbo, Cameroun: first results

The analyses of the stable (deposition and post-depositional) remanent magnetization carried by the lacustrine sediments contained in a 24 m sequence collected from the bottom of the maar lake, Barombi Mbo (Cameroun) yield well-defined declination and inclination oscillations which can be attributed to palaeosecular variations of the local geomagnetic field. The transformation to a time-scale, based on the interpolation and extrapolation of 14C ages, allows comparison of this record with contemporaneous records obtained in other parts of the world. The successive inclination peaks can be individually identified with those found in western Europe, in spite of small chronological discrepancies which are in the range of the standard deviation of the ages. Normalization of the NRMs by anhysteretic magnetization provides a profile of ‘relative palaeointensities’ which shows similarities with that reconstructed from the same time interval in a sequence from western Europe although an important phase shift appears.