Drivers and implications of circulation and evaporite deposition patterns: Modelling the evolution of Messinian Brine

Abstract

The Messinian Salinity Crisis, which led to the formation of the youngest salt giant, has been subject of studies for more than 50 years and happened in a geologically short time span of ca 600 kyr between 5.96 Ma and 5.4 Ma. There are, however, signs that those changes started much earlier, as the sediment markers of the 7.2 event show a seemingly sudden change in conditions. Although there is a stratigraphic consensus model that relates different sediment formations to each other and sorts them into three stages, there is still a plethora of unanswered questions. Their complexity is increased due to the extreme nature of the event which makes it almost impossible to differentiate between noise and signal as well as reworked and deposited sediment. The puzzle, however, is worth solving. The Mediterranean Sea can be regarded as a laboratory ocean and sensitive recorder of global changes, which means that understanding the response of its overturning cell to increase in salinity and changes in freshwater budget will lead to a better understanding of dynamics that are at play now in the global oceans. The Messinian Salinity Crisis thus offers the opportunity to bridge the gap between past and future by adding quantitative interpretations of the event to the qualitative descriptions that already exist. In this work we use box models and mass balance calculations to explore different aspects of this system and interpret their results with a multidisciplinary lens. We apply this method to a prominent deposit in the Balearic Promontory, the timing and distribution of Primary Lower Gypsum and deep basin halite, as well as the dynamics that could have led to the expression of the 7.2 event in the sediment. From this we find that the sedimentary expression at 7.2 might have been caused by an interplay of (1) a slowing down or stopping of the increase in restriction and (2) a change in freshwater budget that occurred around this time. We further confirm that gypsum and halite did not precipitate for an extended amount of time simultaneously in different parts of the basin and propose a timeline of precipitation patterns for a basin increasing in salinity. We also find that gypsum precipitated in unrestricted areas, while the halite deposit in the Central Mallorcan Depression would require a drawdown that disconnected the basin. The implied salinity would be enough to precipitate 90% for the halite deposits in eastern and western basin if the Mediterranean Sea became disconnected and experienced a substantial drawdown. Our results lead us to the conclusion that the Mediterranean was in a transient state during the MSC, which is highlighting the importance of time dependent forcing. I also see the need for a paring of large-scale models spanning the whole basin and event with models that focus on specific aspects. This, in combination with increased collaboration on the structure of models, holds the potential to significantly improve our understanding of this fascinating time in our planet’s past.