The Songolo estuarine swamp (South Congo) during the middle and late Holocene

This paper presents the most recent and detailed Holocene palynological res earch carried out on the Congolese Atlantic littoral. in the region of Poin te-Noire, as part of the ECOFIT program (CNRS-IRD). The results are obtaine d on two continental sedimentary sequences (core S2 from Songolo site :4 de grees 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 C-13 on organic matter) analysis allowing a contin uous reconstruction of the Congolese littoral palaeoenvironments from 7000 yr. cal B.P. onwards. The Congolese coastal plain is today a mosaic of fore st-grassland. Sandy open short savanna largely occurs with as dominant gras s Loudetia arundinacea. Locally, Symphonia globulifera swamp forests are pr esent along rivers and in flooded areas. Mangrove with Rhizophora is not we ll developed except along the Kouilou estuarine. The climate is characteriz ed by mean annual precipitation not exceeding 1300 mm and temperature rangi ng between 22 and 25 degreesC. The core S2 from Songolo site was extracted using a piston corer. The coring site is today a peat building swamp with a bundant Cyperus papyrus. The local dominant trees are Raphia and Alstonia. The presence of Elaeis guineensis (oil palm) and Mangifera indica (mango) t estifies of a recent human occupation of this area. The core Coraf 2 was co llected on the beach by manual penetration of PVC tube. On the two studied cores, dating control is provided by thirteen C-14 conventional and;AMS dat es performed on total organic matter. The ages are reported with equivalent ranges in calendar years. The present pollen, mineralogical and isotopic records, completed by previo us other data (mainly macroflora remains), give new informations on vegetat ion 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 chan ges. The main conclusions can be summarized as follows. During the middle H olocene (7000 - 3000 yr. cal B.P.), expansion of swamp dense forests with P andanus, Anthostema, Hallea, Symphonia globulifera, Syzygium, and also of m angrove (Rhizophora), as testified by pollen analysis, gives evidence of hi gh rainfall which is also supported at Songolo by the abundance of kaolinit e and quartz, an increase in the amorphous silica and delta C-13 low values typical of C-3 biomasse until 3000 yr. cal. B.P. At the same time, the occ urrence in macroflora remains of trees such as Saccoglottis gabonensis Ongo kea 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, co nfirms this climatic interpretation. A major change is registered ca. 3000 yr. cal B.P., characterized by the regression of the swamp forests which ar e replaced by herbaceous formations dominated by Cyperaceae and ferns, the disappearance of mangrove near the Songolo site, an important decrease in m ineral influx (quartz and kaolinite) and increase in organic matter content , Increase in delta C-13 values during this period shows that most of the o rganic fraction derives from C-4; plants. probably linked to local expansio n of Cyperus papyrus. This change, well known at this time on several sites from Central Atlantic Africa, is mainly linked to a progressive regional d eterioration of climate towards aridity. On the Songolo site, the presence of fresh water taxa such as Nymphaea lotus supports an absence of marine wa ter influence, indicating a sea level regressive episode well known, at the same time. along the West African coast. During the most recent period, de spite more favourable climatic conditions (rainfall), no re-extension of li ttoral swamp forests is detected. These results are mainly related to the l ocation of the studied sites, in an area of particularly intense human impa ct 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 Atla ntic Africa (Congo and Cameroon). The general similarity of the registered changes during the last 7000 yr cal B.P, strongly supports a regional clima tic interpretation.