The evolution of oceanic 87Sr/86Sr through the Tertiary has been sufficiently well documented to provide a useful stratigraphic tool for carbonate-rich marine sediments which were deposited in basins connected to the global oceans. In isolated basins, sea water 87Sr/86Sr will diverge from coeval oceanic values; thus if independent age information exists, the geochemical signature can constrain the timing of isolation. In marginal basins formed in tectonically active settings, the timing of isolation is critical to the assessment of the relative importance of climate or tectonic controls on that event. Previously, isolation has been assumed to coincide with the last occurrence of open marine fauna. However, in an example from the Eastern Mediterranean, 87Sr/86Sr data challenge that assumption and can be interpreted as indicating a protracted period of isolation (c. 3 Ma), during which time marine conditions prevailed, prior to the Messinian salinity crisis at the end of the Miocene. This allows tectonic and climatic signals to be distinguished, since although convergence of Africa with Eurasia was responsible for isolation of this part of the Mediterranean, it is more likely that later climatic change triggered desiccation.