What Can Stable Isotopes Say about Salinity? An Example from the Late Miocene Pannonian Lake

Abstract

The mid-to late Miocene Lake was a primarily closed, saline lake which during its lifetime underwent a series of rapid salinity changes associated with faunal extinctions/radiations. The first two of these, at the Badenian/Sarmatian and Sarmatian/Pannonian boundaries signify the transition from a marine basin to a brackish lake and are marked by major faunal changes. The isotopic composition of molluscs from the lake, however, does not undergo major changes. The third transition, at the end of the Pannonian and into the Pontian, is also marked by major faunal changes, thought initially to reflect further freshening of the lake. Both carbon and oxygen isotope ratios of lake molluscs decrease by 4 to 6%oo at this transition. A consideration of salinity and oxygen isotope mass balances for a closed or nearly-closed lake shows that major changes in salinity accompany relatively minor changes in water balance and isotopic composition. This result explains why the oxygen isotope ratio of lake waters did not change during the initial freshening of the lake and suggests that the major isotopic changes observed later are not directly related to changes in the lake water balance which caused the salinity changes. The isotopic variations were driven mainly by other climatic factors, most likely changes in humidity and isotopic composition of inflow.