THE GEOCHEMISTRY OF LAKE BOSUMTWI, A HYDROLOGICALLY CLOSED BASIN IN THE HUMID ZONE OF TROPICAL GHANA

Lake Bosumtwi lies in an isolated, hydrologically closed basin formed by a meteor impact similar to 1 x 10(6) yr ago. Comparison of lake-wat er samples taken in 1993 with one from 1934 shows a decrease in the co ncentrations of dissolved solutes, most likely owing to an estimated 1 3% increase in lake volume. A mass balance calculation that uses an es timate of the average chloride composition of the rainfall shows that the most recent overflow of the lake occurred within the last 3,000 yr . Evaluation of the sources and sinks of major solutes suggests that d eposition of harmattan dust may be an important component of the influ x of ions into the lake. Although the lake is homogeneous with respect to major ion composition, it is stratified into an upper oxygenated l ayer and a lower anoxic layer, and there is an abrupt change in the co ncentration profiles of pH, NH4+, PO43-, NO3-, and Mn at a depth of 16 m. This chemical stratification is apparently caused by photosynthesi s in the upper layer and respiration and reducing conditions below. Th e stable isotopic composition of the lake water is reasonably well sim ulated by a simple isotope balance model for the lake. The model is us ed to show that the delta(18)O composition of the lake is relatively i nsensitive to changes in the water balance, but is highly sensitive to relative humidity and the isotopic composition of atmospheric moistur e.