The Messinian salinity crisis (MSC) - the most abrupt, global-scale environmental change since the end of the Cretaceous – is widely associated with partial desiccation of the Mediterranean Sea. A major open question is the way normal marine conditions were abruptly restored at the end of the MSC. Here we use geological and geophysical data to identify an extensive, buried and chaotic sedimentary body deposited in the western Ionian Basin after the massive Messinian salts and before the Plio-Quaternary open-marine sedimentary sequence. We show that this body is consistent with the passage of a megaflood from the western to the eastern Mediterranean Sea via a south-eastern Sicilian gateway. Our findings provide evidence for a large amplitude drawdown in the Ionian Basin during the MSC, support the scenario of a Mediterranean-wide catastrophic flood at the end of the MSC, and suggest that the identified sedimentary body is the largest known megaflood deposit on Earth.

Carbonate escarpments are submarine limestone and dolomite cliffs that have been documented in numerous sites around the world. Their geomorphic evolution is poorly understood due to difficulties in assessing escarpment outcrops and the limited resolution achieved by geophysical techniques across their steep topographies. The geomorphic evolution of carbonate escarpments in the Mediterranean Sea has been influenced by the Messinian salinity crisis (MSC). During the MSC (5.97-5.33 Ma), the Mediterranean Sea became a saline basin due to a temporary restriction of the Atlantic-Mediterranean seaway, resulting in the deposition of more than one million cubic kilometres of salt. The extent and relative chronology of the evaporative drawdown phases associated to the MSC remain poorly constrained. In this paper we combine geophysical and sedimentological data from the central Mediterranean Sea to reconstruct the geomorphic evolution of the Malta Escarpment and infer the extent and timing of evaporative drawdown in the eastern Mediterranean Sea during the MSC. We propose that, during a MSC base-level fall, fluvial erosion formed a dense network of canyons across the Malta Escarpment whilst coastal erosion developed extensive palaeoshorelines and shore platforms. The drivers of geomorphic evolution of the Malta Escarpment after the MSC include: (i) canyon erosion by submarine gravity flows, with the most recent activity taking place <2600 cal. years BP;(ii) deposition by bottom currents across the entire depth range of the Malta Escarpment;(iii) tectonic deformation in the southern Malta Escarpment in association with a wrench zone; (iv) widespread, small-scale sedimentary slope failures preconditioned by oversteepening and loss of support due to canyon erosion, and triggered by earthquakes. 3We carry out an isostatic restoration of the palaeoshorelines and shore platforms on the northern Malta Escarpment to infer an evaporative drawdown of 1800 -2000 m in the eastern Mediterranean Sea during the MSC. We interpret the occurrence of pre-evaporite sedimentary lobes in the western Ionian Basin as suggesting that either evaporative drawdown and canyon formation predominantly occurred before salt deposition, or that only the latest salt deposition at the basin margin occurred after the formation of the sedimentary lobes.

The Messinian salinity crisis was an extraordinary event that resulted in the deposition of kilometre-thick evaporite sequences in the Mediterranean Sea after the latter became disconnected from the world's oceans. The return to fully and stable marine conditions at the end of the crisis is still subject to debate. Three main hypotheses, based on geophysical and borehole data, onshore outcrops and climate simulations, have been put forward. These include a single-stage catastrophic flood, a two-step reflooding scenario, and an overspill of Paratethyan water followed by Atlantic inflow. In this study, two research questions are addressed: (i) Which event marked the termination of the Messinian salinity crisis?; (ii) What was the sea level in the eastern Mediterranean Sea during this event? Geophysical data from the western Ionian Basin are integrated with numerical simulations to infer that the termination of the crisis consisted of a single-stage megaflood following a sea level drawdown of 1900 m. This megaflood deposited an extensive sedimentary body with a chaotic to transparent seismic signature at the base of the Malta Escarpment. Fine, well-sorted sediments are predicted to have been deposited within the thicker sections of the flood deposit, whereas a more variable distribution of coarser sediments is expected elsewhere. The north-western Ionian Basin hosts evidence of episodic post-Messinian salinity crisis slope instability events in the last ~1.8 Ma.The largest of these emplaced a >200 km 3 deposit and is associated with failure of the head of Noto Canyon (offshore SE Sicily). Apart from unravelling the final phase of the Messinian salinity crisis and the ensuing stratigraphic evolution of the western Ionian Basin, our results are also relevant to better understand megafloods, which are some of the most catastrophic geological processes on Earth and Mars.

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The MARCAN project, launched last January, is working to fill a gap in our knowledge of how freshwater flowing underground shapes and alters the continental margins.

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