ORIGIN AND FUNCTION OF A CLOSED DEPRESSION IN EQUATORIAL HUMID ZONES - THE LAKE TELE IN NORTH CONGO

Lake Tele has a nearly perfect ellipsoidal shape and is surrounded by swamps and barely penetrable flooded forests in the heart of the Congo -Zaire watershed basin, and has intrigued the international scientific community for decades. In June 1992, a first Franco-Congolese multidi sciplinary scientific expedition was finally able to reach the lake wh ich is vast (23 km(2)) and shallow (3 m). Its volume is estimated at 7 1 x 10(6) m(3) and is 40% filled with an organic silt layer a metre th ick, Its hydrological exchanges are almost exclusively vertical with v ery little lateral contribution from the surrounding swamp. This leads to the observation that the waters are very slightly mineralized (< 3 mg l(-1)), but are very rich in organic carbon (44% of suspended matt er and of total dissolved matter) and are very acidic (pH < 4), In add ition, a magnetic anomaly of some hundred nanoTesla from a magnetic bo dy at shallow depth has been detected in the lake's northern half, alt hough its origin remains unknown. Botanical and palynological observat ions suggest the persistence of a strongly hydromorphic forest environ ment for at least the last 6600 years. Identified pollen taxa, whether ancient or recent, come mainly from pioneer and colonizing taxa such as Macaranga, which tends to indicate that the forest is continuing to gradually fill in the lake. (C) 1998 Elsevier Science B.V. All rights reserved.