The Geochemistry and Hydrography of Lake Tanganyika

John Edmond was a key member of the scientific group that participated in the two SIO expeditions on Lake Tanganyika, involved in both the water sampling on the lake and the operations in the potable chemical laboratory used ashore. Much of his data on the nutrient chemistry of the lake has been published in summary form (Limnol.Oceanog.1993). The present paper, which describes some of the other studies made on the lake, is dedicated to John, who was both a close personal friend and a brilliant colleague. Along the ~650 km length of the lake we occupied 18 stations and sampled the major inflowing streams. The CI concentration of the lake waters below ~150m (depth of the epilimnion) is constant (27.8ppm), so that temperature is the only hydrographic variable, and distinctive profiles occur in the two major deep basins (North & South, = NB and SB). In the NB from 100 to 1200m, T° C decreases smoothly to 400m, below which are two ~ isothermal layers extending down to a sharp discontinuity at 700m, followed by a decrease to an nderline{in-situ} minimum (23.32° C at 870m, the coldest water in Lake Tanganyika). The lower-most 300m of the profile is essentially adiabatic, with a bottom T = 23.32° C. The 700m T discontinuity is associated witha sharp cusp in methane concentration, which increases smoothly with depth from zero at the base of the mixed layer to 2.5 cc/kg at 700m, and then increases rapidly to 5.0 cc/kg at 1200m. In the SB, T decreases smoothly to 600m depth, below which is an almost isothermal layer to 1100m, followed by an ~ adiabatic gradient for 300m, to 23.40° C at 1400m. In this basin the CH4 profile is a smoothly continuous curve from 100-1200m, showing that the effective sill-depth between the two basins is at ~700m. Helium isotope profiles also show distinctive profiles in the two deep basins. In the NB, the 4He profile increases downward from atmospheric saturation to a smooth maximum at 450m (2.26 x saturation) and a 3He/4He ratio anomaly δ (3He) = -40% of atmospheric value). In the SB there is a similar though less marked He maximum at 900m. These extrema show the depths of injection of He from crustal sources, which in both basins has a 3He/4He ratio of 0.28 x atmospheric, close to the ratio in radiogenic helium. The He concentration requires a saturation T of 15° C at the present level of 773m above sealevel. If the deep water has not changed and was saturated at the present 23° C, the required lake level is ~250m below the present level. Co2 and 13C data show production of light CO2 at 220m, the depth of a δ (13C) minimum, and on the lake bottom where heavy CO2 is produced by CH4 production. Other data to be discussed as time permits include stable isotopes (D and 18O, enriched in deep water), 14C, tritium, 226Ra, 210Pb, and dissolved N2, Ne, and Ar. Our logistical work was supported by UNDP-FAO. G.W. Coulter (UNDP, Burundi), Ray Weiss (SIO), and Valerie Craig (SIO) participated in the expedition work at sea and on land.