Modes of deep basin halite accumulation during the Messinian Salinity Crisis

Abstract

Hydrological restriction from the Atlantic Ocean and a negative freshwater balance transformed the Mediterranean Sea into a giant saline basin during the Messinian Salinity Crisis (MSC) (5.97 – 5.33 million years ago). After more than 50 years of research, it is still unclear if the deposition of nearly one million km3 of evaporite salts during this event was accompanied by a major (≥ 1.5 km) drawdown of Mediterranean sea level; and if halite deposition occurred only during this drawdown event, or also in a filled Mediterranean connected to the Atlantic.We present evidence based on the chlorine stable isotope composition of halite for a sea level drawdown of 2 km in the eastern Mediterranean during the final stages of deposition of the Mediterranean halite layer. This is the largest sea level drop ever reported from the geological record and implies a short (~20 kyr), but nearly complete hydrological disconnection of the Mediterranean from the Atlantic. About half of the halite volume in the eastern Mediterranean was deposited during this drawdown event. Scant chlorine isotope data from the western Mediterranean, together with new estimates of western basin halite volume, suggest that the complete western halite deposit accumulated during the drawdown event. This mode of halite deposition - termed “evaporative drawdown” - had been already proposed as an evolution of the “deep-basin, shallow-water” scenario for the MSC (Hsü et al., 1973).Chlorine isotopes indicate that the remaining half of the eastern Mediterranean halite volume accumulated prior to the drawdown, in a filled Mediterranean that received a continuous input of Atlantic waters. This halite accumulation phase lasted about 70 kyr and was driven by a restricted - but not fully blocked - outflow of deep western Mediterranean waters to the Atlantic. This model of halite deposition corresponds to the “deep-basin, deep-water” scenario for the MSC (Schmaltz, 1969). In summary, our results show that both the “deep-basin, deep-water” and the “deep-basin, shallow-water” modes of halite accumulation took place during the MSC, and that the complete deep-basin MSC halite deposit accumulated in less than 100 kyr. Hsu, K., Ryan, W. & Cita, M. Late Miocene Desiccation of Mediterranean. Nature 242, 240–244 (1973).Schmalz, R. F. Deep-water evaporite deposition, a genetic model. American Association of Petroleum Geologists Bulletin53, 798–823 (1969).