Injection of mantle type helium into Lake Van (Turkey): the clue for quantifying deep water renewal

Abstract

Helium, neon and tritium concentrations have been measured to study long-term vertical mixing and deep water renewal in Lake Van (Eastern Turkey), the largest soda lake on Earth. Helium excesses were found in the water column of Lake Van, although neon concentrations were close to air saturation. The excesses of both isotopes are strictly correlated and increase with depth. In the bottom water, 4He supersaturation is about 20% and the corresponding 3He concentration is 2.5 times the air saturation value. The mean excess 3He/ 4He ratio, of 1 1.2 · 10 −5, is slightly higher than the MORB ratio found in Lake Nemrut, a neighbouring volcanic crater lake, in which a large input of mantle helium was detected. Mantle helium accounts for the majority of the helium excesses in Lake Van, although part of the 3He 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 reconstruct the evolution of the helium isotopes and the tritium. As a conclusion, the model, based on the measured 3He and 4He profiles, shows that: (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 cm 3 STP · m −2 yr −1 (2–3 · 10 11 atoms · 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 atoms · m −2 s −1; the flux into Lake Van would account for at least 0.04–0.06% of this.