Modeling transport rates in Lake Baikal: Gas exchange and deep water renewal

Concentrations of molecular oxygen in Lake Baikal (Eastern Siberia), the deepest lake on earth (1634 m), are above 80% saturation in the whole water column suggesting fast deep-water renewal. A model is developed to describe vertical water exchange based on measured concentrations of tritium, He-3, and the chlorofluorocarbons CFC-11 (CCl3F) and CFC-12 (CCl2F2). Lake Baikal consists of three main basins (south, central, north) that are separated by sills reaching up to about 300 m depth. Each basin is vertically divided into two boxes: a surface box 300 m thick and a deepwater box below 300 m. Tracers are transported from/into the surface layer by gas exchange with the atmosphere during ice-free periods, by precipitation, by evaporation, and by rivers. The tracer distribution in the lake depends primarily on the three intrabasin vertical water exchange rates. These rates are simultaneously fitted by modeling the four tracers from 1900 to the present. Mean residence times in the south, central, and north basins are 11.2 +/- 0.6, 10.4 +/- 0.5, and 6.2 +/- 0.5 yr, respectively. Application of the transport model to measured oxygen concentrations yields O-2 consumption rates in all deep-water boxes of 0.1 mg L-1 yr(-1).