Mixing and evaporation processes in an inverse estuary inferred from δ2H and δ18O

We have measured δ2H and δ18O in Spencer Gulf, South Australia, an inverse estuary with a salinity gradient from 36‰ near its entrance to about 45‰ at its head. We show that a simple evaporation model of seawater under ambient conditions, aided by its long residence time in Spencer Gulf, can account for the major features of the non-linear distribution pattern of δ2H with respect to salinity, at least in the restricted part of the gulf. In the more exposed part of the gulf, the δ/S pattern appears to be governed primarily by mixing processes between inflowing shelf water and outflowing high salinity gulf water. These data provide direct support for the oceanographic model of Spencer Gulf previously proposed by other workers. Although the observed δ/S relationship here is non-linear and hence in notable contrast to the linear δ/S relationship in the Red Sea, the slopes of δ2H vs. δ18O are comparable, indicating that the isotopic enrichments in both marginal seas are governed by similar climatic conditions with evaporation exceeding precipitation.