The upflow zone of more than 40 cold and hot mineral springs of Na-(Ca)-(Mg)-HCO3-Cl type is restricted to an extensional structure within the seismically active North Anatolian Fault Zone. The chemistry of the waters, which are probably more than 10,000 years old, is influenced effectively in a low-temperature environment by the CO2 originating from the decomposition of marine carbonates and outgassing of the earth mantle. This gas is thus attributed to the interaction of the minor plates of Turkey and the Black Sea in this area. A strong positive O-18-shift is due to water-rock interaction with very long contact times, rather than to isotopic influence of the CO2 on the deltaO-18-value of the waters.