Late holocene hydrological history of Lake Tanganyika, East Africa, from isotopic data on fossil stromatolites

Abstract

Lake Tanganyika, situated in the Western Branch of the East African Rift System, shows a slightly positive water budget and generates a small outflow (through the Lukuga River), dependent mainly on the Ruzizi River inflow from Lake Kivu in the northern part of the basin. Conditions more arid than those presently observed, closed the Kivu Basin from 3800 to 1300 yr B.P. Previous studies esimated that the closure of the corresponding Tanganyika Basin lowered the lake level by 75–150 m. In this paper we present the results of isotopic and morphologic analyses of fossil stromatolites. The stromatolites formed as thick incrustations on hard substrates and are found at depths between 6 and 60 m below the modern lake level. Their isotopic composition ( 18O, 13C, 14C) very precisely document Late Holocene water levels and hydrologic fluctuations in the Lake Tanganyika Basin, with reference to the actual situation. A major low stand, at 10 m below present level, is identified from 3400 to 1900 yr B.P., with a rapid recession and gradual rise to modern levels. Maximum δ 18O values (vs PDB) in the stromatolites rose from ca. 3.5% (at 3400 yr B.P.) to 6% (at 2500 yr B.P.), then decreased progressively to 1.6% (at 1300 yr B.P.). These values are interpreted as a maximum aridity episode culminating at ca. 2500 yr B.P., at which time a negative precipitation/evaporation (P:E) ratio resulted in a much higher residence time of paleolake waters and thus a noticeable enrichment in 18O. The observed decoupling of the isotopic ratio ( 18O/ 16O) and water volume responses is thought to be linked to the inertia of the large rift aquifers which controlled the paleolake level.