Early anthropogenic impact on Western Central African rainforests 2,600 y ago
- aInstitute of Earth and Environmental Science, University of Potsdam, 14476 Potsdam, Germany;
- bAix-Marseille Université, CNRS, IRD, Collège de France, Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement UM34, 13545 Aix-en-Provence, France;
- cUniv Lyon, Ens de Lyon, Université Lyon 1, CNRS, UMR 5276 LGL-TPE, 69342 Lyon, France;
- dPatrimoines Locaux et Gouvernance UMR 208, IRD, MNHN, 75005 Paris, France;
- eMARUM–Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany;
- fNormandie Université, UNIROUEN, UNICAEN, CNRS, M2C, 76000 Rouen, France;
- gHSM, LMI Picass’Eau, IRD, Université de Montpellier, 34095 Montpellier, France;
- hInstitute of Earth Surface Dynamics, Geopolis, University of Lausanne, 1015 Lausanne, Switzerland;
- iAgence Nationale des Parcs Nationaux, 20379 Libreville, Gabon;
- jSection 5.2, Climate Dynamics and Landscape Evolution, GFZ–German Research Centre for Geosciences, 14473 Potsdam, Germany;
- kLaboratoire de Chimie de l’Environnement FRE 3416, Aix-Marseille Université, CNRS, 13545 Aix-en-Provence, France;
- lDepartment of Plant Biology, Faculty of Sciences, University of Yaoundé I, Yaoundé, Cameroon;
- mHerbier National du Cameroun, Institut de Recherche Agricole pour le Développement, Yaoundé, Cameroon;
- nInstitute of Geological and Mining Research, Yaoundé, Cameroon;
- oAlfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, 14473 Potsdam, Germany;
- pSection 5.1, Geomorphology, GFZ–German Research Centre for Geosciences, 14473 Potsdam, Germany
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Edited by Sarah J. Feakins, University of Southern California, Los Angeles, CA, and accepted by Editorial Board Member Ruth S. DeFries January 24, 2018 (received for review August 30, 2017)

Significance
Modern human societies live in strongly altered ecosystems. However, anthropogenic environmental disturbances occurred long before the industrial revolution. About 2,600 y ago, a forest–savannah mosaic replaced dense rainforests in Western Central Africa. This rainforest crisis was previously attributed either to the impact of climate change or, to a lesser extent, to the expansion of Bantu peoples through Central Africa. A 10,500-y sedimentary record from Lake Barombi, Southwest Cameroon, demonstrates that the rainforest crisis was not associated with any significant hydrological change. Based on a detailed investigation of a regional archaeological database, we present evidence that humans altered the rainforest ecosystem and left detectable traces in the sediments deposited in Lake Barombi.
Abstract
A potential human footprint on Western Central African rainforests before the Common Era has become the focus of an ongoing controversy. Between 3,000 y ago and 2,000 y ago, regional pollen sequences indicate a replacement of mature rainforests by a forest–savannah mosaic including pioneer trees. Although some studies suggested an anthropogenic influence on this forest fragmentation, current interpretations based on pollen data attribute the ‘‘rainforest crisis’’ to climate change toward a drier, more seasonal climate. A rigorous test of this hypothesis, however, requires climate proxies independent of vegetation changes. Here we resolve this controversy through a continuous 10,500-y record of both vegetation and hydrological changes from Lake Barombi in Southwest Cameroon based on changes in carbon and hydrogen isotope compositions of plant waxes. δ13C-inferred vegetation changes confirm a prominent and abrupt appearance of C4 plants in the Lake Barombi catchment, at 2,600 calendar years before AD 1950 (cal y BP), followed by an equally sudden return to rainforest vegetation at 2,020 cal y BP. δD values from the same plant wax compounds, however, show no simultaneous hydrological change. Based on the combination of these data with a comprehensive regional archaeological database we provide evidence that humans triggered the rainforest fragmentation 2,600 y ago. Our findings suggest that technological developments, including agricultural practices and iron metallurgy, possibly related to the large-scale Bantu expansion, significantly impacted the ecosystems before the Common Era.
Although the vast rainforests of Western Central Africa (WCA) are not considered pristine ecosystems (1), the influence of humans and climate change on their areal extent and composition during the past remains largely unknown. Over the last three decades, regional pollen sequences recovered from lakes and swamps have provided an unprecedented view of Holocene vegetation changes in the region. The most striking feature is a major vegetation disturbance that occurred between 3,000 calendar years before present (cal y BP) and 2,000 cal y BP (2⇓⇓⇓⇓–7). Although this vegetation disturbance appeared to be widespread across WCA (4), insufficient chronological control and sedimentary hiatuses have limited regional correlations or attribution to a single event (8). There is particularly strong evidence for the late Holocene rainforest crisis (LHRC) recorded in the sediments of Lake Barombi (or Barombi Mbo; 4° 39.6´ N, 9° 24.3´ E; Fig. 1). This sedimentary archive has recorded a reversible switch from a mature rainforest to a disturbed/secondary forest, with a significant proportion of grasses and pioneer trees (3). This event was attributed to aridification associated with an increase in the duration of the dry season (3⇓–5). An abrupt warming of sea surface temperatures (SSTs) in the Gulf of Guinea was inferred to have changed the monsoon precipitation (3, 5). Quantitative pollen-based reconstructions of mean annual precipitation of the Lake Barombi record were interpreted to reflect a 50% decrease in precipitation during the LHRC (9). Contemporaneous dry conditions were also inferred from nearby Lake Ossa using diatoms (10); however, a reanalysis of the sediments from this lake demonstrated a ∼400-y age offset, due to the influence of aged soil carbon, and a dominant control of sedimentation by local forcing factors rather than regional climate, which have cast doubt on this assessment (11).
Map of the Lake Barombi basin with topography, bathymetry (10-m contour intervals), and the coring site. Also highlighted are infrastructures and farmland/highly degraded forest based on SPOT 5 images and ArcGIS Online Basemap–World Imagery (source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community). (Inset) Location map of Lake Barombi (yellow star) in Africa.
Although the LHRC is contemporaneous with a marked increase in human settlements and activity in the region (7, 12⇓⇓⇓–16), it is generally assumed that humans at that time were not yet major agents of vegetation disturbances (4, 13, 15, 17⇓⇓–20). A recent study (21) suggesting that human land-use intensification associated with the southward migration of Bantu-speaking farmers across Central Africa contributed to forest destruction has, however, fueled a fierce debate in the paleoecology and archaeology communities (22, 23).
To support the notion of a climatic, i.e., hydrological driver of the LHRC, it is necessary to demonstrate independently that vegetation and hydrological changes were coeval, which has not been confirmed to date. Alternatively, to support anthropogenic impacts, changes in hydrological and vegetation proxies should be disconnected and unambiguous evidence of coeval human activity should be demonstrated.
Here we make use of the capability of stable carbon and hydrogen isotope ratios of plant wax-derived biomarkers to record both vegetation and hydrological changes independently in the same organic molecule (24, 25). We obtained a new 12-m-long sediment core from Lake Barombi (105-m water depth, Fig. 1) spanning the past 10,500 cal y BP. Our well-constrained age model is based on a total of 35 14C dates and on 210Pb assay for the top of the sequence (SI Appendix). Paired carbon and hydrogen isotope compositions of the n-C31 alkane (
Records of environmental change from Lake Barombi over the past 10,500 y and comparison with local summer insolation and Gulf of Guinea SSTs. (A) Long-term insolation changes at 5°N during Northern Hemisphere summer [June–July–August (JJA)] (40). (B) Lake Barombi δ
Results
In Africa,
In contrast to δ13C, which is a proxy indicator for vegetation, δ
Discussion
The Lake Barombi plant wax record indicates a D enrichment during the Holocene interpreted as a drying signal. No such drying trend was evident from former pollen-based precipitation reconstructions from this site (9). This low-frequency variability suggests a dominant control of the Northern Hemisphere summer insolation on the isotopic composition of monsoon precipitation at orbital timescales (Fig. 2A). The timing of the drying at Lake Barombi differs from that at Lake Bosumtwi (6° 30´ N) in West Africa, which recorded a reduction in precipitation from 9,000 cal y BP to 5,500 cal y BP, suggesting a regionally variable precipitation response to solar radiation (38).
In the Lake Barombi area, modern precipitation reaches ∼2,500 mm⋅y−1, which is sufficient to enable rainforest growth. Therefore, the vegetation remained relatively insensitive to the drying trend during the Holocene. Although late Holocene drier conditions led to increasing sensitivity to disturbances and a sustained switch from forest to savannah at the periphery of the former rainforest (15, 44, 45), the absence of any coeval drying (i.e., increase in δ
Our results call for a reappraisal of the ‘‘anthropogenic hypothesis’’ (2, 21). To support this hypothesis, we reconstructed the temporal patterns of population dynamics and cultural change in WCA using a newly compiled database of 460 archaeological sites and 1,202 14C dates covering the last 10,500 y derived from ref. 47 (Figs. 3 and 4; see SI Appendix for detailed information). Before 4,000 cal y BP, dated archaeological sites were scarce and disparate in the study area. In contrast, coeval with the onset of the LHRC at Lake Barombi at 2,600 cal y BP, the number of dated archaeological sites increased abruptly (Fig. 3B). Based on the summed probability distribution (SPD) of calibrated 14C dates from this database (Fig. 4C), we inferred a significant change (at 95% confidence level) in the archaeological record, highlighting a major positive deviation in population dynamics that occurred during the LHRC and that may be related to the Bantu expansion in WCA (7, 13, 19, 48⇓–50). Starting from 3,000 cal y BP, sedentary Neolithic populations making pottery and carrying out pit features settled in WCA (13). At ∼2,400 cal y BP, these populations began to cultivate pearl millet (
Map of WCA, the Lake Barombi study site, and spatiotemporal distribution of 460 archaeological sites covering the past 10,500 y. (A) Current spatial distribution of rainforest shown in green, taken from the Collection 5 Moderate Resolution Imaging Spectroradiometer (MODIS) Global Land Cover Type product (www.landcover.org). (Inset) Map showing location of the study area in Africa. Countries: AGO, Angola; CAR, Central African Republic; CMR, Cameroon; COG, Republic of the Congo; DRC, Democratic Republic of the Congo; GAB, Gabon; GNQ, Equatorial Guinea; NGA, Nigeria. Overlain are 14C-dated archaeological sites with dated and associated material as well as the location of Lake Barombi and marine core MD03-2707. (B) Number (No.) of 14C-dated archaeological samples per 100-y period. Vertical yellow band indicates the timing of the Lake Barombi LHRC.
Detailed comparison of the Lake Barombi vegetation changes and the archaeological record of WCA during the past 5,000 y. (A) Lake Barombi
We note that the inception of the LHRC as recorded at Lake Barombi occurred ∼200 y earlier than the apparent establishment of technological developments in the whole of WCA (Fig. 4B). This difference in timing might either reflect chronological uncertainties or be an evidence of diffusion of people and/or technological developments related to the initial stage of the supposed expansion of Bantu-speaking peoples in WCA (51, 52) and associated disturbances.
On a local scale, our data suggest that humans practiced land clearing in the Lake Barombi basin during a major positive deviation in population dynamics associated with technological developments occurring at a regional scale that resulted in a local rainforest decline. The subsequent abandonment of the catchment or switch to agricultural practices with reduced impacts on vegetation could have fostered the regeneration of the rainforest.
Although humans probably triggered the LHRC in WCA, climate might have had an indirect role: In the drier regions north of the rainforest, concurrent late Holocene drying coupled with land degradation (56) may have encouraged populations to migrate southward, disseminating new technological developments into the rainforest.
Our results provide additional temporal and causal constraints on the current models of Bantu expansion—based on human genetic and linguistic phylogenies (19, 50, 57)—suggesting that the LHRC was the consequence rather than the cause of human dispersal in Central Africa.
Materials and Methods
This section presents the site description and a summary of methods, with full details provided in SI Appendix.
Study Site and Regional Climate.
Lake Barombi is a 4.2-km2, 105-m-deep, and circular (diameter of ∼2,300 m) lake filling a volcanic maar crater. It is located 334 m above sea level, ∼35 km to the northeast of Mt. Cameroon. It is an oligotrophic freshwater open lake overflowing into the Kumba River. Lake surface temperatures range from 26.7 °C to 29.5 °C (58). Below ∼20 m the lake develops an anoxic hypolimnia (58). The Barombi maar is a polygenetic structure that formed through three eruptive cycles between ∼510 kya and 80 kya (59). Phreatomagmatic explosions west of the Barombi maar resulted in the formation of an adjacent maar crater (59), which comprises most of the lake catchment area (10.4 km2) and is drained by a network of small perennial rivers. The lake occupies the Barombi maar to the east, which is bowl shaped with steep slopes and a flat bottom. The local vegetation comprises lowland evergreen rainforest with patches of semideciduous forest (3, 60). A forest reserve was created in 1940 to protect the vegetation in the vicinity of the lake (61). Since ∼30 y ago human impact related to farming and deforestation in the lake catchment strongly accelerated (62). The farming practice relies on mixed crops such as cocoyam, plantain, and maize and monocrop systems such as cocoa, oil palm, and rubber (61). Regional climate is controlled by the seasonal variability of the intertropical convergence zone (ITCZ) and its associated rain belt, which brings monsoonal precipitation from March to October. The mean annual rainfall at Kumba (town bordering the Lake Barombi catchment) is ∼2,500 mm⋅y−1, with monthly precipitation less than 100 mm⋅mo−1 during 3–4 mo and a mean monthly temperature of 27 ± 2 °C (63).
Sediment Core.
In January 2014 core B14 was collected in the deepest (105 m deep) and central part of Lake Barombi from a UWITEC hybrid floating platform using a UWITEC percussion piston coring system. Based on previous studies of the Lake Barombi sediments (3, 30, 64, 65) we focused our analyses on the first 12 m of sediments, which represent the Holocene Epoch. A continuous 11.71-m-long composite sequence was constructed from holes B14-2 and B14-5 after core splitting based on lithological features (SI Appendix, Fig. S1). Event deposits (turbidites and tephras) were excised from the original composite depth scale, resulting in an event-free 9.94-m-long depth scale. Based on 21 210Pb dates and 35 14C dates (SI Appendix, Tables S1–S3) we generated an age model for the Lake Barombi event-free sediment record (SI Appendix, Figs. S2 and S3) using Bayesian approaches, as implemented in the freely available R statistical computing package Bacon v2.2 (66, 67) and using the Southern Hemisphere calibration curve SHCal13 (68).
Molecular and Isotopic Analyses.
Core B14 was sampled continuously with contiguous 1-cm slabs for the topmost 25 cm of the sequence and contiguous 5-cm slabs for the rest of the sequence. Components of terrestrial plant waxes were extracted from freeze-dried lacustrine sediments using an accelerated solvent extractor with a dichloromethane/methanol mixture of 9:1. The n-alkanes were isolated over a solid phase extraction on silica gel and then purified. They were identified by gas chromatography mass spectrometry using a flame ionization detector and quantified by comparison with an internal standard and standard mixtures of n-C10-n-C40 alkanes. Carbon and hydrogen isotope ratios (SI Appendix, Fig. S4) were measured using gas chromatography coupled to an isotope ratio mass spectrometer, using a combustion interface (for δ13C) and a pyrolysis furnace (for δD). Carbon and hydrogen isotope values were corrected based on values of isotopic standards and are reported on the Vienna Pee Dee Belemnite and Vienna Standard Mean Ocean Water scales, respectively.
Acknowledgments
We thank the Barombi people and their Chief J. S. Ndokpe as well as the administrative authorities of Cameroon for support and collaboration during fieldwork. We thank T. Klaka-Tauscher and M. Elomo Molo (logistics); R. Niederreiter (core collection); M. Pöhle, M. Patyniak, X. Hadeen, and T. Syla (sample processing); G. Zeilinger (help with GIS analysis); B. Ghaleb (210Pb analyses); and E. R. Crema (help with SPD modeling of calibrated 14C dates). We also thank an anonymous reviewer and K. Uno for their helpful comments on the manuscript. Fieldwork was conducted with research permission of the Ministry of Scientific Research and Innovation of Cameroon. SPOT satellite images (©CNES) were acquired through the ISIS-888 Project. We acknowledge the French National Research Institute for Sustainable Development (IRD), its local office in Yaoundé, and the LMI DYCOFAC for their support. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) (Grant GA1629/2 to Y.G.). Y.G. and M.R.S. were also supported by the Ministry of Science, Research, and Culture des Landes Brandenburg. P.D. was supported by Labex OT-Med and EQUIPEX ASTER-CEREGE projects. G.M. acknowledges the BNP-Paribas Climate Initiative Foundation (Project CPATEMP). E.S. and L.M.D. were supported by the DFG-Research Center/Cluster of Excellence “The Ocean in the Earth System” at MARUM–Center for Marine Environmental Sciences.
Footnotes
- ↵1To whom correspondence should be addressed. Email: yannickgarcin{at}yahoo.fr.
Author contributions: Y.G. designed research; Y.G. coordinated research; Y.G., P.D., G.M., B.B., K.G.M., and A.A.A. performed fieldwork; Y.G. conducted sample preparation, chemical sample treatment, and stable hydrogen isotope analysis; E.S. carried out stable carbon isotope analysis; Y.G., G.d.S., and R.O. compiled and analyzed archaeological data; L.M.D. carried out pollen analysis; Y.G., P.D., G.M., G.d.S., E.S., D. Sebag, L.M.D., R.O., B.B., K.G.M., J.-M.O., A.A.A., L.S.E., R.T., M.R.S., A.B., and D. Sachse contributed to the interpretation and the discussion of the results; and Y.G. wrote the paper with contributions from all authors.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. S.J.F. is a guest editor invited by the Editorial Board.
Data deposition: The data reported in this paper have been deposited in the PANGAEA online database (https://doi.pangaea.de/10.1594/PANGAEA.884676).
See Commentary on page 3202.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1715336115/-/DCSupplemental.
Published under the PNAS license.
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