How dry was the Messinian Salinity Crisis? - a molecular biogeochemical study of the Eraclea Minoa (Sicily) section, Italy -
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The Messinian Salinity Crisis (MSC; 5.96-5.33 Ma) is considered one of the most enigmatic episodes of paleo-oceanographic change during the Cenozoic era. Kilometres thick evaporites are deposited in the Mediterranean basin, during periods when the connections between the Atlantic Ocean and the Mediterranean were restricted. The development through time of this crisis is still under debate. Although it is generally accepted that the MSC was a dry period with higher evaporation than precipitation and runoff, how dry climate was and how saline the water, has not yet been quantified accurately. Samples from the Upper Evaporites (MSC stage 3; 5.50 - 5.33 Ma) were collected from the Eraclea Minoa section (Sicily), consisting of cyclic alternations of gypsum and marls. The main aim of this study is to reconstruct changes in the hydrological cycle during Upper Evaporite deposition. The hydrogen isotopic distribution is closely related to the hydrological cycle and build into organic molecules. This offers the opportunity to reconstruct past changes in the hydrological cycle and salinity using compound specific hydrogen isotopic analyses. The δD of terrestrial n-alkanes (C25 – C31) mainly records the δD of precipitation, modified by site specific meteorologic conditions and evapotranspiration in leaves. The δD of long chain ketones, produced by haptophyte algae, mainly depends on the δD of the seawater, but also on salinity and to some degree growth rate. Both long chain n-alkanes with an odd over even predominance (higher plants), short chain marine n-C17 and long chain ketones were found, recording heavy (deuterium enriched) hydrogen isotopic values. The very heavy surface water values are in line with exceptional high rates of evaporation. The Uk’37 values based on the same long chain ketones suggest overall relatively high SST (albeit at the maximum of the available calibration range) which, together with extremely dry conditions might have contributed to the extreme evaporation rates. Still, the presence of long chain ketones and absence of the C37:4 ketone in the Upper Evaporites suggests that connections between the Atlantic and Mediterranean, despite being reduced, were also open during stage 3 of the MSC. The reconstructed hydrogen isotopic composition of the precipitation is dependent on the fractionation factor used. The deuterium enriched precipitation values in a temperate (European) climate setting could be the result of the more heavy hydrogen isotopes from the seawater being reflected in local rain on the Mediterranean coast, albeit that admixing of marine organic matter contributed to the heavy values as well. However, the hydrogen isotopic composition of precipitation in a dry climate setting are comparable to present-day values and reconstructed salinities average around 65 g/l, which suggests a dilution of the Mediterranean surface water and a stratification of the water column. This stratification could explain the formation of gypsum deposits in brine circumstances in the lower water column and survival of open ocean haptophyte algal species in the upper water column. Overall, high SST appear to coincide with the deposition of marls and sandstones at the start of the cycle and massive gypsum in the upper part of the cycles. However, hydrogen isotopic values are most deuterium enriched in the massive gypsum deposits where most evaporation takes place during an insolation minimum, while the most depleted values occur in the marls at the start of the cycle coinciding with an insolation maximum and a high freshwater input. More measurements are needed to study these small scale changes in hydrogen isotopic values and their relation to climatic changes in detail.