INJECTION OF MANTLE TYPE HELIUM INTO LAKE VAN (TURKEY) - THE CLUE FORQUANTIFYING DEEP-WATER RENEWAL

Helium, neon and tritium concentrations have been measured to study lo ng-term vertical mixing and deep water renewal in Lake Van (Eastern Tu rkey), the largest soda lake on Earth. Helium excesses were found in t he water column of Lake Van, although neon concentrations were close t o air saturation. The excesses of both isotopes are strictly correlate d and increase with depth. In the bottom water, He-4 supersaturation i s about 20% and the corresponding He-3 concentration is 2.5 times the air saturation value. The mean excess He-3/He-4 ratio, of 1.2 . 10(-5) , is slightly higher than the MORB ratio found in Lake Nemrut, a neigh bouring volcanic crater lake, in which a large input of mantle helium was detected. Mantle helium accounts for the majority of the helium ex cesses in Lake Van, although part of the He-3 excess is attributed to the presence of tritium. A one-dimensional lake model was used, which describes tritium input, vertical mixing and gas exchange, to reconstr uct the evolution of the helium isotopes and the tritium. As a conclus ion, the model, based on the measured He-3 and He-4 profiles, shows th at: (1) the vertical exchange of deep water occurs within 1-2 yr; (2) the flux of mantle helium into the lake, averaged over the total cross -section of Lake Van, is 0.23-0.35 cm3 STP . m-2 yr-1 (2-3 . 10(11) at oms . m-2 s-1). The present estimate of the global mantle helium flux averaged over the total surface area of the Earth is < 3.9 . 10(9)) at oms m-2 S-1; the flux into Lake Van would account for at least 0.04-0. 06% of this.