Diatom-derived palaeoconductivity estimates for Lake Awassa, Ethiopia: evidence for pulsed inflows of saline groundwater?

A 6,500-year diatom stratigraphy has been used to infer hydrochemical chang es in Lake Awassa, a topographically closed oligosaline lake in the Ethiopi an Rift Valley. Conductivity was high from similar to 6400-6200 BP, and fro m 5200-4000 BP, with two brief episodes of lower conductivity during the la tter period. Although the timing of the conductivity changes is similar to the timing of lake-level change in the nearby Zwai-Shalla basin, their dire ctions are the reverse of that expected from a climatic cause. Dissolution of the tephras which precede both phases of high conductivity cannot explai n the increases in salinity, because rhyolitic tephras are only sparingly s oluble. Instead, the pulsed input of groundwater made saline by the reactio n of silicate minerals and volcanic glass with carbonic acid, formed from t he solution of carbon dioxide degassed from magma under the Awassa Caldera, is suggested as a plausible mechanism for the observed change in lake chem istry. Diatom-inferred hydrochemistry cannot therefore be used to reconstru ct climate change in Lake Awassa.