Charophytes are found in some ephemeral saline lakes in Australia. Healthy green charophytes, bearing oogonia, are frequently observed at salinities ca. 1-2 times that of seawater. Field observations of salinity tolerance are confirmed by experiments which demonstrate photosynthetic 14 C-carbon dioxide fixation at these salinities. The lakes containing these charophytes have various hydrological settings but are all influenced by inputs of continental groundwater of seasonally varying salinity. Different evaporite minerals are deposited depending upon local desiccation-evaporation balances. In coastal lagoons adjacent to the Coorong, charophytes are found associated with calcite, aragonite and protodolomite while in the continental lakes around northern Spencer Gulf the observed associations are with gypsum and halite. Lake deposits containing charophyte oogonia and discoidal gypsum provide a recent analog for some ancient evaporite units which are not adequately explained by the arid, coastal marine evaporite model based on the present-day Persian Gulf sabkhas. For example the Purbeckian evaporites of Dorset (England) and the Tertiary evaporites of the Paris Basin (France) both contain charophyte and evaporite-bearing units which are similar to the deposits of these Recent Australian lakes. We suggest that sediments containing associations of charophyte oogonia and evaporites formed as a result of seasonally varying salinity within lakes of the semi-arid, Mediterranean-type, climatic zone.

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