Early anthropogenic impact on Western Central African rainforests 2,600 y ago

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

Imaging of non-traumatic abdominal emergencies in adults at Limbe Regional Hospital: A qualitative survey of frontline physicians

Introduction: The choice of imaging modalities in non-traumatic abdominal emergencies typically manifesting as pain is informed by the clinical context, availability, cost and safety considerations. The objective of this study was to elucidate how emergency department (ED) physicians use and perceive the role of imaging in non-traumatic abdominal emergencies (NTAE) in adults. Materials and methods: A qualitative in-depth interview of consenting physicians working at the ED of Limbe Regional Hospital was conducted during the month of April 2021. Data was collected on sociodemographic variables, work experience and imaging prescription attitudes for non-traumatic abdominal emergencies. Interview data were audio-recorded, transcribed and analyzed thematically. Results: Fourteen participants were interviewed in-depth of which 9 (64.3%) were females and the median age (range) of all participants was 29 (27 to 35) years. The average duration of work experience after qualification as a physician was 2 years (range: 1 to 6 years) and all participants were general practitioners. Plain abdominal radiography and ultrasonography were reported to be the first-line imaging modalities for all NTAEs, with both often requested sequentially. The utilization of computed tomography was reserved for situations where plain films and ultrasonography were negative or indeterminate, and often upon recommendation by a specialist physician or the reporting radiologist. The perceived role of imaging was to provide a diagnosis as the clinical impression was often uncertain. Conclusion: Plain abdominal films and ultrasonography remain key imaging modalities for the assessment of non-traumatic abdominal emergencies, with computed tomography utilization being more reserved. The continuous sensitization and mentoring of frontline physicians will be useful to foster better utilization of imaging modalities.   English title: Imagerie des urgences abdominales non traumatiques chez l’adulte : Enquête qualitative auprès des médecins du service des urgences à Hôpital Régional de Limbe  Introduction: La disponibilité croissante des modalités d'imagerie dans les hôpitaux donne la possibilité aux médecins de première ligne de choisir la technique la mieux indiquée pour leurs patients. L'objectif de cette étude était d'élucider comment les médecins du service des urgences de l’Hôpital Régional de Limbe au Cameroun conçoivent l’utilité des modalités d'imagerie médicale dans la prise en charge des urgences abdominales non traumatiques chez les adultes. Matériel et méthodes: Un entretien qualitatif détaillé avec les médecins exerçant au service des urgences était réalisé pendant le mois d’avril 2021. Les données étaient recueillies sur les caractéristiques sociodémographiques et les habitudes lors des demandes d’examen d’imagerie pour investiguer les urgences abdominales non-traumatiques. Les interviews étaient enregistrées sur fichier audio, transcrit et analysé de façon thématique. Résultats: Quatorze participants ont été interrogés dont 9 (64,3%) étaient des femmes, et l'âge médian (intervalle) était de 29 (27 à 35) ans. La durée moyenne de l'expérience de travail après l’obtention du diplôme de docteur en médecine était de 2 ans (intervalle : 1 à 6 ans). Tous les participants étaient des médecins généralistes. L'échographie et la radiographie de l’abdomen sans préparation, souvent couplée, étaient citées comme les modalités d'imagerie demandées en première intention pour investiguer les urgences abdominales non traumatiques. La demande de la tomodensitométrie était réservée aux cas où l’échographie et la radiographie étaient non contributives, voire par recommandation par un médecin spécialiste ou du radiologue. Selon les répondants, le principal rôle de l'imagerie serait de fournir un diagnostic de certitude. Conclusion: L'échographie et la radiographie demeurent les principales modalités d'imagerie dans l'investigation des urgences abdominales non-traumatiques, l’utilisation du scanner étant plus restreinte. Une sensibilisation pour renforcer les connaissances et pratiques des médecins de première ligne s’avérait utile

Hydrogen isotope ratios of lacustrine sedimentary n-alkanes as proxies of tropical African hydrology: Insights from a calibration transect across Cameroon

Hydrogen isotope values ( δ D) of sedimentary aquatic and terrestrial lipid biomarkers, originating from algae, bacteria, and leaf wax, have been used to record isotopic properties of ancient source water (i.e., precipitation and/or lake water) in several mid- and high-latitude lacustrine environments. In the tropics, however, where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes, calibration studies for this proxy are not yet available. To close this gap of knowledge, we sampled surface sediments from 11 lakes in Cameroon to identify those hydro-climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers. Here we present a robust framework for the application of compound-specific hydrogen isotopes in tropical Africa. Our results show that the δ D values of the aquatic lipid biomarker n -C 17 alkane were not correlated with the δ D values of lake water. Carbon isotope measurements indicate that the n -C 17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis. We demonstrate that the δ D values of the n -C 29 alkane were correlated with the δ D values of surface water (i.e., river water and groundwater), which, on large spatial scales, reflect the isotopic composition of mean annual precipitation. Such a relationship has been observed at higher latitudes, supporting the robustness of the leaf-wax lipid δ D proxy on a hemispheric spatial scale. In contrast, the δ D values of the n -C 31 alkane did not show such a relationship but instead were correlated with the evaporative lake water δ D values. This result suggests distinct water sources for both leaf-wax lipids, most likely originating from two different groups of plants. These new findings have important implications for the interpretation of long-chain n -alkane δ D records from ancient lake sediments. In particular, a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources. Our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and/or growth form of organisms and therefore, the source of hydrogen used during lipid biosynthesis

n-alkane carbon isotope ratios in plants and recent lake sediments from Cameroon

Trees and shrubs in tropical Africa use the C3 cycle as a carbon fixation pathway during photosynthesis, while grasses and sedges mostly use the C4 cycle. Leaf-wax lipids from sedimentary archives such as the long-chain n-alkanes (e.g., n-C27 to n-C33) inherit carbon isotope ratios that are representative of the carbon fixation pathway. Therefore, n-alkane d13C values are often used to reconstruct past C3/C4 composition of vegetation, assuming that the relative proportions of C3 and C4 leaf waxes reflect the relative proportions of C3 and C4 plants. We have compared the d13C values of n-alkanes from modern C3 and C4 plants with previously published values from recent lake sediments and provide a framework for estimating the fractional contribution (areal-based) of C3 vegetation cover (fC3) represented by these sedimentary archives. Samples were collected in Cameroon, across a latitudinal transect that accommodates a wide range of climate zones and vegetation types, as reflected in the progressive northward replacement of C3-dominated rain forest by C4-dominated savanna. The C3 plants analysed were characterised by substantially higher abundances of n-C29 alkanes and by substantially lower abundances of n-C33 alkanes than the C4 plants. Furthermore, the sedimentary d13C values of n-C29 and n-C31 alkanes from recent lake sediments in Cameroon (-37.4 per mil to -26.5 per mil) were generally within the range of d13C values for C3 plants, even when from sites where C4 plants dominated the catchment vegetation. In such cases simple linear mixing models fail to accurately reconstruct the relative proportions of C3 and C4 vegetation cover when using the d13C values of sedimentary n-alkanes, overestimating the proportion of C3 vegetation, likely as a consequence of the differences in plant wax production, preservation, transport, and/or deposition between C3 and C4 plants. We therefore tested a set of non-linear binary mixing models using d13C values from both C3 and C4 vegetation as end-members. The non-linear models included a sigmoid function (sine-squared) that describes small variations in the fC3 values as the minimum and maximum d13C values are approached, and a hyperbolic function that takes into account the differences between C3 and C4 plants discussed above. Model fitting and the estimation of uncertainties were completed using the Monte Carlo algorithm and can be improved by future data addition. Models that provided the best fit with the observed d13C values of sedimentary n-alkanes were either hyperbolic functions or a combination of hyperbolic and sine-squared functions. Such non-linear models may be used to convert d13C measurements on sedimentary n-alkanes directly into reconstructions of C3 vegetation cover

Reply to Clist et al.: Human activity is the most probable trigger of the late Holocene rainforest crisis in Western Central Africa

(IF 9.58; Q1)International audienc

Reply to Giresse et al.: No evidence for climate variability during the late Holocene rainforest crisis in Western Central Africa

(IF 9.58; Q1)International audienc

Plant-waxes hydrogen and carbon isotopic composition and pollen data of sediment core B14 (Lake Barombi)

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. d13C-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. dD 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