Multiple crises preceded the Mediterranean Salinity Crisis: Aridification and vegetation changes revealed by biomarkers and stable isotopes

摘要

During the Messinian (7.24-5.33 Ma), the highly dynamic Mediterranean environment was concomitantly governed by global climate changes and regional tectonic activity affecting the connectivity to the global ocean. The combined effects generated extreme and rapid paleoenvironmental changes culminating in the Messinian Salinity Crisis (MSC; 5.96-5.33 Ma). Here, we reconstruct paleoenvironmental conditions recorded in the Agios Myron section (Crete Island, Greece) between-7.2 and 6.5 Ma that indicate marked changes affecting the eastern Mediterranean region prior to the onset of the MSC. Hydrogen isotope ratios measured on alkenones produced by haptophyte algae within the Mediterranean water column, coupled to carbon isotopes measured on long chain n-alkanes produced by higher terrestrial plants show three important dry periods peaking at 6.98, 6.82 and 6.60 Ma accompanied by shifts in vegetation, transitioning from dominantly C3 to markedly increased C4 plants contribution and intermittent recurrence of C3 vegetation at-6.99 Ma and 6.78 Ma. Mean annual air temperatures reconstructed using branched glycerol dialkyl glycerol tetraethers (brGDGTs) average 13 degrees C with an overall pattern that permits orbitally-controlled pacing of the regional climate. Additionally, the branched and isoprenoid tetraether index and bulk carbon and oxygen isotope ratios indicate changes in the source(s) of organic matter and evolution of the basin towards a closed and arid system. These results support a model of ongoing restriction affecting the eastern Mediterranean Sea from-7 Ma onwards and reveal a protracted ari-dification of the Mediterranean domain prior to the onset of the MSC.