| Popis: |
Magnesium silicates such as chain clays and Mg-rich interstratifications commonly form in saline and alkaline soil and lake waters. Geochemical analyses of water and sediment from the Lake Eyasi and Ngorongoro Crater watersheds (northern Tanzania) were carried out to test the hypothesis that elevated aqueous CO 2 due to biological activity strongly affects the stability of Mg-rich clay minerals. Field data and model calculations show that P CO2 is an important control on pH, thereby affecting mineral stability. Initial supersaturation of water with respect to magnesium clays generally requires elevated pH, and subsequent pH suppression due to biotic CO 2 can prevent mineral precipitation. At Ngorongoro and Eyasi, solute Na + , SO 4 2− , Cl − , and alkalinity follow generally conservative evaporative enrichment trajectories, whereas solute Ca 2+ is lost to calcite, SiO 2 to biogenic amorphous silica, and K + to clay uptake. Mg 2+ behavior is highly variable, and does not follow any simple trajectory. Minimal evaporative concentration is required for magnesium silicate supersaturation, and a strong correlation is found between P CO2 and magnesium silicate solubility (Ngorongoro r 2 =0.70, p 2 =0.61, p 2 gas equilibrium status. These diagenetic processes should be considered in the use of lacustrine clay minerals as paleochemical indicators, because diagenetic pH suppression in sub-lacustrine pore waters can effectively remove indicators of high paleo-salinity |
