GEOTHERMAL ACTIVITY AS A KEY TO THE PAST: STABLE ISOTOPES INDICATE THE PRESENCE OF MESSINIAN EVAPORITES IN THE SUBMARINE HYDROTHERMAL SYSTEM OF PANAREA, ITALY R. Sieland 1* , C. Müller 2 , M. Tichomirowa 2 , K. Knöller 3 , M. Schipek 1 , B. Merkel 1 1 Scientific Diving Center, D-09596 Freiberg, GERMANY *Corresponding author: Robert.Sieland@geo.tu-freiberg.de 2 TU Bergakademie Freiberg, Institute of Mineralogy, D-09596 Freiberg, GERMANY 3 Helmholtz Centre for Environmental Research - UFZ, D-06120 Halle, GERMANY Submarine volcanic activity occurring about 2.5 km east of Panarea Island is linked to the back-arc volcanism in the Tyrrhenian Sea (Aeolian Islands, Italy). Fumarolic gas and thermal waters discharge from a submerged volcanic crater in a water depth between 8 and 30 m. Water and rock samples were taken by scuba divers between 2006 and 2012 in order to study geological and chemical processes for volcanic risk analysis. The chemical composition of fluid discharges differs significantly within distances of few hundred meters. Stable isotopic signatures give insights into the origin and formation of hydrothermal fluids as well as the potentially occurring interaction, separation, mixing or cooling processes. Isotopic analyses on hydrothermal water samples were conducted for δD water , δ 18 O water and 87 Sr/ 86 Sr. Additionally, rock samples were analysed for 87 Sr/ 86 Sr. The isotopic composition of δD/δ 18 O in fluid samples indicates a mixture between magmatic water, Mediterranean seawater and an isotopic light component probably caused by meteoric water. However, hydrothermal fluid samples show a positive δD-shift in comparison with the Local Meteoric Water Line for the Mediterranean Sea [1]. This might arise from strong water- rock-interactions in the underground leading to isotopic-exchange reactions. During the Messinian Salinity Crisis (6.4 to 4.85 Ma) a sequence of evaporates some hundreds of meters thick had been deposited [2] in the nowadays Mediterranean Sea region. The upper evaporates consist of massive gypsum whose hydrate water is characterized by δD ~ -36.9±2.0‰ and δ 18 O ~ -3.7±0.65 ‰ [3]. Hydrothermal fluid samples with lowest seawater proportion around 30% (indicated by 2-component mixing calculations based on 87 Sr/ 86 Sr data) plot close-by a line intersecting the gypsum hydrate water of the Messinian (R² = 0.965, p < 0.0005). Thus, the dehydration of Messinian gypsum layers favored by an increased thermal gradient [4] related to geothermal activity might be a source of the isotopic light component. 87 Sr/ 86 Sr ratios of Panarea fluids indicate a two-component mixing (R²=0.952, p < 0.0005) between ambient seawater (0.7092) and another component (0.7065). However, rock samples from volcanic islets surrounding the submerged hydrothermal crater are characterized by lower 87 Sr/ 86 Sr ratios around 0.7058. Therefore, the second component might arise from a mixture between unweathered deep basalt rock ( 87 Sr/ 86 Sr: 0.7029 to 0.7058) and Messinian gypsum deposits (0.7084 – 0.7065, [5]). Volcanic activity in the Panarea-Stromboli system started about 800 ka ago [6] and is significantly younger than the Messinian Salinity Crisis in Miocene times. Hence, volcanism had to penetrate Messinian evaporate deposits. [1] LIOTTA, M. et al. (2008) Journal of Geophysical Research 113, 1-10 [2] MÜLLER, D.W., MÜLLER, P.A. (1991) Earth and Planetary Science Letters 107, 1-12 [3] BELLANCA, A., NERI, R. (1986) Journal of Sedimentary Petrology 56, 614-21 [4] Testa, G., Lugli, S. (2000) Sedimentary Geology 130, 249-268 [5] RAAB, M. ET AL. (1997) Carbonates and Evaporites 12, 296-324 [6] DE ASTIS, G. ET AL. (2003) Tectonics 22 (4), 14-1 – 14-17