HYDROMAN: Impacts of hydrological changes versus human activity on Central-West African forests during the Holocene
Mineralogy, Petrology and Geochemistry
Final Report Abstract
In the context of the current climate change and its societal impacts, it is essential to understand the role of humans and their activity on the environments and the possible consequences on climate. The debate concerning (i) the formalization of the “Anthropocene” into a new geological epoch—implying that the influence of Man on the biosphere is such that it has become the dominant geological constraint and (ii) the designation of its starting influence, is highly controversial. Until now, the beginning of major human influences on the environment and its potential effects on the climate was generally associated with the beginning of the industrial revolution. A recent study by Bayon et al. published in the journal Science in 2012 proposed a human influence on the environments causing the fragmentation of the forests of Central Africa (termed the rainforest crisis) about 2500 years ago, i.e. well before the industrial age. This interpretation has been strongly disputed in the scientific community and it is clearly recognized that an abrupt climatic change was at the root of this deforestation. To support a climatic origin of this forest crisis, it is necessary to demonstrate independently that vegetation and climate changes were contemporaneous, which has not been confirmed so far. Alternatively, to support an anthropogenic origin, changes in climatic and vegetation proxies must be disconnected and unequivocal evidence of contemporary human activity must be provided. Within the frame of this project, I proposed to test these hypotheses and try to unravel the enigma of the rainforest crisis. I suggested using the stable carbon and hydrogen isotopes of sedimentary lipid biomarkers derived from plant waxes that have the capacity to record both vegetation and hydrological changes independently on the same organic molecule. For this, with an international team of scientists we re-cored Lake Barombi (or Barombi Mbo, 4° 39.6'N, 09° 24.3'E) in Southwest Cameroon in 2014 (it had already been cored in 1985 but the sediment had since disappeared). Three parallel sediment cores approximately 20-m-long each were collected at a depth of 105 m and were used to build a complete composite core. The dating of the composite core is based on 35^14C ages. The stable carbon and hydrogen isotopic compositions were measured on the n-C31 alkane on 220 samples. In parallel to the geochemical study of the lake core, together with archaeologists working in the region for decades we have created a regional archaeological database for Central Africa referencing all existing archaeological sites and their 14C dates. The carbon isotope composition of plant waxes from the sediments of Lake Barombi confirmed the existence of the forest crisis but contrary to the hypotheses put forward so far, the hydrogen isotope composition of these same plant waxes, which reflects hydrological changes, yielded no indication of a coincident shift in climate. Based on the analysis of our archaeological database, we have observed that archaeological remains older than 3000 years were rare in Central Africa. Around 2600 years ago, coincident with the rainforest crisis at Lake Barombi, the number of sites increased significantly suggesting a rapid human population growth. This period also saw the emergence of pearl millet cultivation, oil palm use, and iron metallurgy in the region. The combination of regional archaeological data and our results from the sediments of Lake Barombi suggests that humans impacted the tropical forests of Central Africa thousands of years ago, and left detectable anthropogenic footprints in geological archives.
Publications
- 2018, Early anthropogenic impact on Western Central African rainforests 2,600 y ago: Proceedings of the National Academy of Sciences USA, vol. 115, 3261– 3266
Garcin, Y., Deschamps, P., Ménot, G., de Saulieu, G., Schefuß, E., Sebag, D., Dupont, L., Oslisly, R., Brademann, B., Mbusnum, K.G., Onana, J.-M., Ako, A.A., Epp, L.S., Tjallingii, R., Strecker, M.R., Brauer, A., Sachse, D.
(See online at https://doi.org/10.1073/pnas.1715336115)
