Le marais estuarien de la Songolo (Sud Congo) a l'Holocene moyen et recent

This paper presents the most recent and detailed Holocene palynological research carried out on the Congolese Atlantic littoral, in the region of Pointe-Noire, as part of the ECOFIT program (CNRS-IRD). The results are obtained on two continental sedimentary sequences (core S2 from Songolo site: 4 degrees 45'51"S, 11 degrees 51'55"E, alt. 5 m and core 2 from Coraf site: 4 degrees 45'S, 11 degrees 51'E, alt. 1 m). Pollen data and interpretation are completed on the Songolo sequence by mineralogical (quartz, kaolinite, TOM) and isotopic (delta 13 C on organic matter) analysis allowing a continuous reconstruction of the Congolese littoral palaeoenvironments from 7000 yr. cal B.P. onwards. The Congolese coastal plain is today a mosaic of forest-grassland. Sandy open short savanna largely occurs with as dominant grass Loudetia arundinacea. Locally, Symphonia globulifera swamp forests are present along rivers and in flooded areas. Mangrove with Rhizophora is not well developed except along the Kouilou estuarine. The climate is characterized by mean annual precipitation not exceeding 1300 mm and temperature ranging between 22 and 25 degrees C. The core S2 from Songolo site was extracted using a piston corer. The coring site is today a peat building swamp with abundant Cyperus papyrus. The local dominant trees are Raphia and Alstonia. The presence of Elaeis guineensis (oil palm) and Mangifera indica (mango) testifies of a recent human occupation of this area. The core Coraf 2 was collected on the beach by manual penetration of PVC tube. On the two studied cores, dating control is provided by thirteen 14 C conventional and AMS dates performed on total organic matter. The ages are reported with equivalent ranges in calendar years. The present pollen, mineralogical and isotopic records, completed by previous other data (mainly macroflora remains), give new informations on vegetation and hydrological changes at local and regional scale since 7000 yr. cal B.P. on the Congolese littoral, in relation to climatic and sea level changes. The main conclusions can be summarized as follows. During the middle Holocene (7000-3000 yr. cal B.P.), expansion of swamp dense forests with Pandanus, Anthostema, Hallea, Symphonia globulifera, Syzygium, and also of mangrove (Rhizophora), as testified by pollen analysis, gives evidence of high rainfall which is also supported at Songolo by the abundance of kaolinite and quartz, an increase in the amorphous silica and delta 13 C low values typical of C 3 biomasse until 3000 yr. cal. B.P. At the same time, the occurrence in macroflora remains of trees such as Saccoglottis gabonensis, Ongokea gore and several species of Monopetalanthus that are found today in the Mont de Cristal forest in Gabon, under 2000-2500 mm of annual rainfall, confirms this climatic interpretation. A major change is registered ca. 3000 yr. cal B.P., characterized by the regression of the swamp forests which are replaced by herbaceous formations dominated by Cyperaceae and ferns, the disappearance of mangrove near the Songolo site, an important decrease in mineral influx (quartz and kaolinite) and increase in organic matter content. Increase in delta 13 C values during this period shows that most of the organic fraction derives from C 4 plants, probably linked to local expansion of Cyperus papyrus. This change, well known at this time on several sites from Central Atlantic Africa, is mainly linked to a progressive regional deterioration of climate towards aridity. On the Songolo site, the presence of fresh water taxa such as Nymphaea lotus supports an absence of marine water influence, indicating a sea level regressive episode well known, at the same time, along the West African coast. During the most recent period, despite more favourable climatic conditions (rainfall), no re-extension of littoral swamp forests is detected. These results are mainly related to the location of the studied sites, in an area of particularly intense human impact as shown by the occurrence of Elaeis guineensis (oil palm) in macroflora remains recovered from numerous archeological sites. The multidisciplinary data presented here correlate well with previous records from Central Atlantic Africa (Congo and Cameroon). The general similarity of the registered changes during the last 7000 yr cal B.P. strongly supports a regional climatic interpretation.