**3.3.2 Oxygen (O)**

The present-day 18O/16O (δ18O) ratio of sulphates in oceanic water reaches 9.5±0.5‰ with respect to V-SMOW (Longinelli & Craig, 1967) but during crystallization of the oceanic sulphates, the δ18O is raised up to 3.5‰ (Lloyd, 1968; Pierre, 1988) and δ18O value of this sulphates reaches 13.0±0.5‰.

Primary gypsum and its crystallization water are formed in isotopic equilibrium with the mother brine (Sofer, 1978), but gypsum can easy loose its original crystallization water during further dehydration and hydration. During hydration sulphates interact with meteoric-, ground-, residual or sea water and gypsum absorbs this new, fresh or sometimes mixed primary water. In the areas of several-, several dozen of m long profiles consisting gypsum rocks, basing on the determination of δ18O of their crystallization water, it is possible to indicate the type and range of individual water types which affected the sulphates. E.g. in profiles of the cap-rock of the Wapno and Mogilno salt diapirs (Jaworska, 2010) there is gypsum, which shows δ18O of crystallization water indicating the influence of: cold period post-glacial water – δ18O reaches values from -11 up to -13‰ in the lowest part of the profile (Wapno and Mogilno), recent (or similar to) meteoritic water - δ18O reaches values of -9 to - 10‰ (Wapno), cap-rock water - δ18O reaches -4.3 to -6.6‰ (Mogilno), "mixing" water or warmer period water - δ18O is -5.6‰ (Wapno) and from -6.9 to -8.7‰ (Mogilno).

The presence of water described as recent or originated from the colder periods inside the lowest and the middle parts of the cap-rock is very important for further management plan of such salt structure. The influence of present day water or the water from colder periods in the lowest part of the cap-rock indicates free flow of surface water into the area of so called salt mirror; the presence of this water in the middle part of the cap-rock indicates the occurrence of cracks, fractures and karst forms in cap-rock body. In consequence it means, that such cap-rock is not a hermetic cover and does not fulfil the requirements for a seal which protects the rock salt and salt mirror against inflow of freshwater. This information is of great importance for salt structures which are prepared for underground disposal of radioactive waste or for the storage of hydrocarbons, as well as salt mine.

#### **3.3.3 Strontium (Sr)**

The 87Sr/86Sr ratio of modern oceanic water is uniform and reaches 0.70901 (Burke et al., 1982) but has been changing in time. Main reasons of these irregular changes were contribution of Sr with high 87Sr/86Sr ratios from continents and input of Sr with low 87Sr/86Sr ratios from active mid –oceanic ridges (Veizer, 1989; Chaudhuri & Clauer, 1992). The general trends and variations of the marine Sr isotopes during the Phanerozoic carbonates are known (Burke et al., 1982) and this curve (the same as S-curve) allows us to study the age of evaporates precipitation. In evaporites the 87Sr/86Sr ratios reflect the isotopic composition of the brines or diagenetic fluids. Strontium does not fractionate (Holster, 1992).

Present-day strontium isotope ratio equilibrated between 87Sr-depleted young oceanic basalts and hydrothermal activity along mid-oceanic ridges (ca. 0.7035) and 87Sr-enriched continental sediments (from old continental granites) transported into the basin by wind and rivers (ca. 0.7119 and more; Chaudhuri & Clauer, 1992; Dickin, 2005). It is the same reason why primary Sr isotopic ratio of evaporites could not be the same as that of contemporaneous sea water – e.g. sediments may have deposited in closed basin with inflow of continental water and continental Sr - the Sr ratio of such sulphates is higher than the one of contemporaneous ocean water, so any variation of Sr isotopic composition may relate to the paleohydrology of the basin. Additionally, variations of Sr isotopic ratio may be explain by contamination with more radiogenic Sr or by diagenesis (Hess et al., 1986; Saunders et al., 1988; Chaudhuri & Clauer, 1992).
