Ilham Bentaleb

Climatic and cultural changes in the west Congo Basin forests over the past 5000 years

Central Africa includes the worlds second largest rainforest block. The ecology of the region remains poorly understood, as does its vegetation and archaeological history. However, over the past 20 years, multidisciplinary scientific programmes have enhanced knowledge of old human presence and palaeoenvironments in the forestry block of Central Africa. This first regional synthesis documents significant cultural changes over the past five millennia and describes how they are linked to climate. It is now well documented that climatic conditions in the African tropics underwent significant changes throughout this period and here we demonstrate that corresponding shifts in human demography have had a strong influence on the forests. The most influential event was the decline of the strong African monsoon in the Late Holocene, resulting in serious disturbance of the forest block around 3500 BP. During the same period, populations from the north settled in the forest zone; they mastered new technologies such as pottery and fabrication of polished stone tools, and seem to have practised agriculture. The opening up of forests from 2500 BP favoured the arrival of metallurgist populations that impacted the forest. During this long period (2500–1400 BP), a remarkable increase of archaeological sites is an indication of a demographic explosion of metallurgist populations. Paradoxically, we have found evidence of pearl millet (Pennisetum glaucum) cultivation in the forest around 2200 BP, implying a more arid context. While Early Iron Age sites (prior to 1400 BP) and recent pre-colonial sites (two to eight centuries BP) are abundant, the period between 1600 and 1000 BP is characterized by a sharp decrease in human settlements, with a population crash between 1300 and 1000 BP over a large part of Central Africa. It is only in the eleventh century that new populations of metallurgists settled into the forest block. In this paper, we analyse the spatial and temporal distribution of 328 archaeological sites that have been reliably radiocarbon dated. The results allow us to piece together changes in the relationships between human populations and the environments in which they lived. On this basis, we discuss interactions between humans, climate and vegetation during the past five millennia and the implications of the absence of people from the landscape over three centuries. We go on to discuss modern vegetation patterns and African forest conservation in the light of these events.

Tracking land-cover changes with sedimentary charcoal in the Afrotropics

Fires have played an important role in creating and maintaining savannas over the centuries and are also one of the main natural disturbances in forests. The functional role of fires in savannas and forests can be investigated through examining sedimentary charcoal in order to reconstruct long-term fire history. However, the relationship between charcoal and vegetation structure in tropical grassy ecosystems remains to be elucidated. Here, we compared recent charcoal records from lake sediments in three tropical ecosystems (forest, savanna, and forest–savanna mosaic) with land cover inferred from remote-sensing images. Charcoal width-to-length (W/L) ratio is a good proxy for changes in fuel type. At one of the lakes, a significant W/L modification from values >0.5 (mainly wood) to <0.5 (~grass) was recorded simultaneously with changes in land cover. Indeed, a significant deforestation was recorded around this lake in the remote-sensing imagery between 1984 and 1994. The results also indicate that a riparian forest around a lake could act as a physical filter for charcoal accumulation; we used the mean charcoal size as a proxy to evaluate this process. Charcoal Accumulation Rates (CHAR), a burned biomass proxy, were combined with W/L ratio and the mean charcoal size to investigate the land-use history of the landscapes surrounding the study sites. This combined approach allowed us to distinguish between episodic slash-and-burn practices in the forest and managed fields or pastures burning frequently.

New Evidence of Human Activities during the Holocene in the Lowland Forests of the Northern Congo Basin

In the last decade, the myth of the pristine tropical forest has been seriously challenged. In central Africa, there is a growing body of evidence for past human settlements along the Atlantic forests, but very little information is available about human activities further inland. Therefore, this study aimed at determining the temporal and spatial patterns of human activities in an archaeologically unexplored area of 110,000 km 2 located in the northern Congo Basin and currently covered by dense forest. Fieldwork involving archaeology as well as archaeobotany was undertaken in 36 sites located in southeastern Cameroon and in the northern Republic of Congo. Evidence of past human activities through either artifacts or charred bo- tanical remains was observed in all excavated test pits across the study area. The set of 43 radiocarbon dates extending from 15,000 BP to the present time showed a bimodal distribution in the Late Holocene, which was interpreted as two phases of human expansion with an intermediate phase of depopulation. The 2300-1300 BP phase is correlated with the migrations of supposed farming populations from northwestern Cameroon. Between 1300 and 670 BP, less material could be dated. This is in agreement with the population collapse already reported for central Africa. Following this, the 670-20 BP phase cor- responds to a new period of human expansion known as the Late Iron Age. These results bring new and extensive evidence of human activities in the northern Congo Basin and support the established chronology for human history in central Africa.

Foraging fidelity as a recipe for a long life: foraging strategy and longevity in male Southern Elephant Seals.

Identifying individual factors affecting life-span has long been of interest for biologists and demographers: how do some individuals manage to dodge the forces of mortality when the vast majority does not? Answering this question is not straightforward, partly because of the arduous task of accurately estimating longevity in wild animals, and of the statistical difficulties in correlating time-varying ecological covariables with a single number (time-to-event). Here we investigated the relationship between foraging strategy and life-span in an elusive and large marine predator: the Southern Elephant Seal (Mirounga leonina). Using teeth recovered from dead males on îles Kerguelen, Southern Ocean, we first aged specimens. Then we used stable isotopic measurements of carbon () in dentin to study the effect of foraging location on individual life-span. Using a joint change-point/survival modelling approach which enabled us to describe the ontogenetic trajectory of foraging, we unveiled how a stable foraging strategy developed early in life positively covaried with longevity in male Southern Elephant Seals. Coupled with an appropriate statistical analysis, stable isotopes have the potential to tackle ecological questions of long standing interest but whose answer has been hampered by logistic constraints.

Variations in 14C Reservoir Ages of Black Sea Waters and Sedimentary Organic Carbon during Anoxic Periods: Influence of Photosynthetic Versus Chemoautotrophic Production

Radiocarbon activity of dissolved inorganic carbon has been measured in the northwestern Black Sea. Both continental shelf and open-sea profiles show that surface waters are in equilibrium with the atmosphere. The observed distribution of 14C activity shows a weak contribution of the deep 14C-depleted CO2 to the photic zone. Such a distribution of 14C within the water column is unable to explain the aging of sedimentary organic matter and reservoir ages greater than 500 yr. A contribution of production by chemoautotrophic bacteria feeding on 14C-depleted methane at the boundary of the oxic and anoxic zones is a realistic hypothesis. Also, a contribution to sedimentary organic carbon estimated at <15% of the photosynthetic primary production could explain 14C reservoir ages greater than 1300 yr.

Coupled Rock-Eval pyrolysis and spectrophotometry for lacustrine sedimentary dynamics: Application for West Central African rainforests (Kamalété and Nguène lakes, Gabon)

In recent years, Nguène Lake and Kamalété Lake (Gabon, West Central Africa) have been studied repeatedly, providing comprehensive reconstructions of environmental changes over the last millennia. Both lakes are in different geomorphological and environmental settings. They are therefore excellent sites to test new methodological approaches. Indeed, the sedimentary cores provide various facies, and the previous studies provide references for calibrating the results of new methods. In this methodological issue, the present study aims to evaluate the potential of spectrophotometric and Rock-Eval coupled analysis to describe the Holocene lake and marsh deposits from tropical moist forests. This assessment is carried out on samples taken from two well-documented reference cores. The spectrophotometric analysis provides reproducible colour measurements, which inform about the nature of the main colour-bearing constituents. Coupled with Rock-Eval pyrolysis, this technique can be used to describe lithological changes and identify the probable source of sedimentary organic matter. In the studied cases, this approach identified the facies dominated by detrital terrigenous inputs (‘iron bearing’ signature and high OI values) and those associated with a more abundant primary production (‘chlorophyll’ signature, low OI and high HI), providing a distinction between palustrine and lacustrine dynamics. However, although the facies are comparable, sedimentary dynamics and sediment sources may vary depending on geomorphological and climatic contexts.

Cloud dynamic contribution to high-elevation peatland growth during the Holocene (Escalerani, Central Andes, Bolivia):

Changes in climate conditions during the Holocene are documented in different parts of South America, showing contrasting responses to global changes. This study was conducted in the wet puna at an elevation of 4040 m a.s.l. on the eastern side of the Cordillera Real in Bolivia near Lake Titicaca. Pollen, charcoal, and stable isotopes in a sediment core collected in the peatland of Escalerani were analyzed. Results revealed environmental changes during the past 7500 yr BP, with an increase in wet climate conditions from 5900 to 4700 cal. yr BP and 3500 to 1300 cal. yr BP, and two dry periods between 4700 and 3500 cal. yr BP and 1300 to 560 cal. yr BP. Changes in hydrological conditions ranged from local changes because of glacier melting to regional changes in annual rainfall variability, related to South American monsoon activity. Moreover, our results highlight the importance of cloud convective activity from the Amazon basin along the adiabatic gradient, which maintained moist conditions at high elevations even during the mid-Holocene dry phase. The last 70 years have been characterized by the degradation of the peatland because of human activity.