The carbon and oxygen isotopic composition of seventy-nine samples of biogenic carbonates from the Mainz Basin Tertiary (Oligocene and Lower Miocene) was analysed. Most samples were mollusc shells still consisting of aragonite. Assuming only small temperature effects, salinity trends derived from isotope data are consistent with palaeontological results from the region: a salinity cycle ranging from fresh water-brackish (Lower Oligocene) towards marine (Middle Oligocene) and brackish-fresh water (Upper Oligocene) was found. Within the Lower Miocene, a trend of decreasing salinities is suggested. Though the isotopic salinity trends coincide rather well with palaeontological salinities, the absolute oxygen isotope ratios indicate an unusual isotopic environment enriched in 18O.

Isotope fractionation is explained by evaporation of a closed basin (Rupelton excluded) with fresh water influx from surrounding land areas in a subtropical climate. Enrichment in 18O by repeated evaporation processes is paralleled by increasing concentration of Sr. Increasing fresh water influx during the Oligocene is due to climatic changes with a trend of more humid conditions towards the younger rock strata.