Carbonate speleothems from gypsum caves for palaeoclimatic and palaeoenvironmental reconstructions

International audienceCarbonate speleothems record a series of environmental and climatic information (McDermott, 2004). Their mere presence in a cave signifies that water was present in the karst system and conditions were adequate to stimulate supersaturation with respect CaCO3. The most exploited geochemical proxies are oxygen (δ18O) and carbon (δ13C) stable isotopes. Furthermore, speleothems are precisely datable. This permits to accurately constrain the time period(s) during which the cave system was capable of producing speleothems and to insert the proxies data into a rigid timeframe. Although speleothems have occupied a key role in “palaeo” science over the last twenty years (Henderson, 2006), there is a striking gap in the literature. All previous studies have been based on carbonate speleothems coming from limestone/dolostone karst. The potential of speleothems sampled in non-calcareous caves for palaeoclimate and palaeoenvironmental research is essentially unexplored. In this work we present several applications of the study of speleothems sampled in gypsum caves (Northern Apennines). The U-Th dating of five speleothems grew in the Monte Tondo karst system allowed to evaluate the climate- driven incision/aggradation of the Senio valley at intra-Milankovitch timescale. This study suggests that speleogenetic processes were active during relative cold stages, when the local base level stabilized a new altitudinal position and the scarcity of vegetation on the valley slopes favored the flux of sediments forming the river terraces. On the contrary, speleothems grew during periods of relative warm and wet conditions over the last 125,000 years, and their dating gives the minimal age of the cave where they were found. Furthermore, speleothems were sampled in five karst system nearby Monte Tondo; twenty more samples were dated with the U-Th method (~100 ages produced) and analyzed for δ18O and δ13C. Our results show that 1) the production of speleothems from gypsum caves responds to strict climatic and environmental parameters; 2) the stable isotopic composition is influenced by the amount of rainfall reaching the cave- site at the time of the formation of the speleothems. Interestingly, some of the speleothems reported an unexpected old age. This permitted the chronological revision of the inception of the speleogenetic activity in the Northern Apennines that, until this date, was considered to be occurred during the last 130,000 years. Caves were instead forming at least 450 ka, ~350 kyrs after the regression of the Adriatic Sea. This lapse was necessary for the erosion of most of the sediments covering the gypsum beds and the maturation of the drainage network, which facilitated karst infiltration in trough sinkholes and blind valleys