Taphonomic bias on calcareous micro and nannofossils and paleoenvironmental evolution across the Messinian Salinity Crisis onset: Insights from the Sorbas Basin (SE Spain).

During the Messinian Salinity Crisis (MSC, 5.97-5.33 Ma) the synergy between tectonic and climatic processes promoted the deposition of evaporites throughout the Mediterranean Basin. The disappearance of calcareous nannofossils and foraminifers at the MSC onset has been traditionally interpreted as the result of the establishment of hypersaline conditions in the water column, which were lethal for most marine eukaryotes. Here, we investigate the calcareous micro and nannofossil assemblages from the Sorbas Basin (SE Spain) spanning the onset and the first phase of the MSC (similar to 5.9-5.7 Ma) in order to test whether their disappearance was related to adverse environmental conditions and/or to preferential dissolution of biogenic calcite. Micropaleontological analyses and petrographic observations suggest that the disappearance of calcareous fossils was the result of a taphonomic bias related to paleoceanographic changes that favoured the oxidation of both pyrite and organic matter at the sea floor. A poorly preserved benthic and planktic assemblage and aragonitic laminated sediment characterize the deposits of the first phase of the MSC from 5.93 Ma onward; aragonite deposition was likely biomediated in response to enhanced photosynthesis, as suggested by its delta C-13 signature. The fossil assemblage preserved in these deposits indicates a shallowing upward trend of the basin with respect to the pre-evaporitic phase, and the establishment of a lagoon-like environment characterized by episodic improvement of the connections with the open sea. We conclude that the temporary disappearance of calcareous fossils approximately at the MSC onset was related to ongoing basin restriction, which increased the sensitivity of the basin to fresh-water input and temperature changes, promoting marked fluctuations of the redox conditions in bottom sediments. This finding challenges the view of a marine biotic crisis related to hypersaline conditions at the MSC onset and opens new perspectives on the paleoenvironmental conditions that characterized this time interval.