Isotope contents, Cl/Br ratio and origin of water at Pikrolimni Lake: A natron source in Greece, as archive of past environmental conditions

The origin of Pikrolimni water was investigated in this study, using stable isotopes, to confirm Pliny’s description that “chalastraion nitron” was produced in the basin of Lake Pikrolimni (Northern Greece). Lake Pikrolimni is located in the basin of Kilkis plain, northern Greece, formed in a shallow depression. Lake Pikrolimni is not fed by any major river, and the hydrography of the area shows that no perennial streams enter the lake directly. In the past, the major source of water to the lake was ground water. Today the dominant inflow to Lake Pikrolimni is meteoric water. The meteoric origin of the water is one of the necessary geochemical conditions that are indispensable and responsible for the formation of carbonate-bicarbonate sodium (Natron, Trona) minerals. Pikrolimni brines lie on an evaporation trend given by δD = 4.74‰ and δ18O = −13.38‰. Both isotopic values (δ18O and δD) increase with salinity, which in turn varies considerably with flooding and evaporation. In dry seasons, part of the lake is desiccated and the margins are locally covered by salts, including Na-carbonate-bicarbonate minerals, Na-sulfate and Na-chloride minerals. These minerals are submitted to repeated wetting and drying cycles under alkaline and strong evaporating conditions. This salt dissolution is reflected in Cl/Br ratios of Pikrolimni water.