The Messinian on Gavdos (Greece) and the status of currently used ages for the onset of the MSC and gypsum precipitation

摘要

The Upper Tortonian and Messinian in the already classic Metochia section on Gavdos Island (Greece) is made up of deep marine, laminated and homogeneous marls with the latter being replaced by evaporitic limestones at the very top of the section. Previous tuning of the section to the La90 time series of northern summer insolation provided ages for sedimentary cycles, polarity reversals, and bioevents. Earlier work on foraminifers and stable isotopes focused on the last 2.45 Myr of the Tortonian and the first 470 kyr of the Messinian. This study extends the stable isotope record and the set of semi-quantitative data on planktonic foraminifers into the youngest 820 kyr of the section which include the precursor and initial stages of the Messinian Salinity Crisis (MSC). New and old foraminiferal oxygen and carbon isotope data have been combined into a similar to 25 kyr resolution record for the entire pre-evaporitic Messinian and show a major change at 6.74 +/- 0.04 Ma which we interpret to reflect an abrupt change in salinity from normal marine to hypersaline and thus marks the prelude to the MSC. Field and thin-section observations on the limestones in the top of the section confirm their evaporitic origin and suggests that salinities has risen to values of >70 psu. We further discuss the biotic response to the two-step salinity increase during the Messinian. Tuning provides an age of 6.00 Ma for the base of the first evaporitic limestone on Gavdos and this age is therefore the age for the onset of the MSC in this part of the Mediterranean. An evaluation of the published ages for the MSC onset in sections Perales (SE Spain), Monticino (N Italy), and Falconara (Sicily) shows that the onset in Falconara, just as on Gavdos, begins at similar to 6.00 Ma with calcium carbonate precipitation, whereas in Perales and Monticino, onset begins at 5.97 Ma (similar to the currently used age of 5.971 Ma) with gypsum precipitation, i.e. some 30 kyr later than the onset of the MSC on Gavdos and Sicily (Falconara).