OVERLOOKED ROLE OF EARLY DIAGENESIS DURING THE FORMATION OF THE MESSINIAN (LATE MIOCENE) SALT GIANT IN THE MEDITERRANEAN

During the late Miocene, the Mediterranean basin was turned into the youngest Salt Giant of Earth history. In less than 650,000 years, more than 1 million km³ of evaporitic minerals (mostly gypsum and halite) accumulated in both marginal and deep Mediterranean sub-basins. Due to the absence of modern analogues, the causes and consequence of the so-called Messinian salinity crisis (MSC; 5.97-5.33 Ma) are still being debated. A long-held assumption is that the establishment of evaporitic conditions led to the annihilation of Mediterranean biota, as demonstrated by the substantial lack of a rich fossil record preserved in the sedimentary successions deposited during the crisis. However, the scattered evidence pointing to the persistence of life in the Mediterranean basin during the MSC might represent much more than exceptions. Actually, they might underline a complex scenario typified by the systematic diagenetic overprint at the expense of the original biogenic signal, resulting in the paucity of well-preserved evidence of the Mediterranean biota. Surprisingly, the possible role played by early diagenesis in controlling the preservation of organic matter and biominerals in Messinian sediments has been often omitted from the debate. Here we present selected case studies aimed at showing the effect of diagenesis on the biogenic content of fine-grained sediments and gypsum crystals formed during the MSC. In the first case, we stress the relevance of dolomite and authigenic clays as remarkable mineralogical proxies of the very early diagenetic alteration of organic matter and associated biominerals, notably biogenic silica, promoted by bacterial sulfate reduction under bottom water anoxic conditions. These were favored by the combination of basin restriction and increasing influence of riverine runoff, resulting in water column stratification. Such a scenario is supported by both inorganic (carbon isotopes) and organic (lipid biomarkers) geochemical evidence. Recent investigations on the state of preservation of solid inclusions in gypsum crystals might suggest the intervention of similar processes. The results strengthen the hypothesis of a diagenetic bias affecting the MSC record and highlight the role of bacterially-mediated degradative pathways of biogenic remains during Salt Giant formation.