Stable isotope composition of the Miocene Dinaride Lake System deduced from its endemic mollusc fauna

Abstract

The aragonite shells of 55 mollusc specimens from the late Early and early Middle Miocene of two palaeolakes of the Dinaride Lake System (DLS) are analysed for their δ18O and δ13C signatures. The data set has a bimodal distribution with a prominent peak between −3 and −4‰ for both isotopes and a second much weaker peak at more depleted values of c. −9 to −7‰. Taxa with ‘heavy’ values are interpreted to represent the inhabitants of the shores or shallow areas of the lake. Depleted values are attributed to species preferring freshwater habitats of springs or small rivulets, thus representing the freshwater end-member. The overall rather high values in both stable isotopes result from a combination of evaporation effects and the influx of already isotopically heavy freshwater from the karstic catchment area. Thus, the DLS is interpreted as a closed freshwater system without marine influence but with enough contribution of freshwater to keep the oxygen values close to −5‰. Mean lake surface water temperatures, based on the δ18Oshell values, excluding the freshwater end-member taxa, range around roughly 19–21°C. Despite the problems in calculating these temperatures, the values are in agreement with mean annual air temperatures deduced from the palaeobotanic record by applying the Coexistence Approach method. The seasonal range was around 10–11°C and is mainly recorded in shells from lake settings. Taxa with freshwater end-member signatures lack this range due to the more stable temperature regime and less fluctuating dissolved inorganic carbon (DIC) pool. Congeneric species from the same stratigraphic horizon often display discrete but different isotope signatures. This fact suggests habitat segregation and might be a major factor for sympatric speciation and the endemic radiation within the mollusc fauna of the DLS.