Between Scylla and Charybdis (part 1): the sedimentary dynamics of the modern Messina Strait (central Mediterranean) as analogue to interpret the past

The present paper reviews a number of geophysical and geological datasets acquired in recent and less recent times on the modern Messina Strait, aiming at reconstructing the sedimentary dynamics of this complex system. In a second paper, the results achieved with the present study are used as proxy for interpreting the analogue lower Pleistocene succession cropping out along the margins of the modern strait. The tide-dominated Messina Strait separates the Italian peninsula and Sicily in the central Mediterranean Sea. This 3-km-wide marine passageway is governed by bi-directional tidal currents flowing along the strait axis, because of semi-diurnal tidal phase opposition, so that high tide in the Ionian Sea corresponds with low tide in the Tyrrhenian Sea and vice versa. Tidal currents cyclically accelerate passing through the strait central restriction and decelerate towards the strait exits, following reverse directions during each tidal phase. This hydrodynamics generates specific by-pass areas, sediment routes and accumulation patterns in the modern strait, based upon the varying bed-shear stress exerted onto the strait bottom by the converging/diverging M2 tidal waves. Recent multibeam-based investigations on the modern Messina Strait bottom reveal subaqueous morpho-bathymetric features, which allow a number of areas with specific morphologic characters and sedimentary processes to be identified: (i) the ‘strait-centre zone’ is the narrowest and shallowest by-pass sector, where the tidal currents reach their maximum strength eroding and scouring the substrate. (ii) Towards NE and S, two larger ‘dune-bedded zones’ occur. Here, clastic sediments transported as bed load and in suspension are accumulated and continuously reworked forming characteristic bedform fields, whose features reflect complex current patterns due to a strong tidal asymmetry. (iii) The ‘strait-end zones’ represent distal depositional areas, where decelerating currents rework fine-grained smaller moribund dunes. (iv) The ‘strait-margin zones’ are diffusely characterised by mass-wasting processes along the steepest sublittoral sectors, where sediment instability are caused by earthquakes or sea storms. Fan-deltas prograde from the gentler-sloping coastal segments, represent the main sediment entry points with episodic, high-magnitude discharge and are influenced by the tidal circulation only along their delta-front parts. However, a dogmatic zone partition, based on a ‘mutually-evasive’ mechanism of sediment distribution and consequent decrease in the transport capacity towards the strait ends is not everywhere applicable to the Messina Strait sedimentary dynamics. The examination of measured tidal ellipses reveals that although flood and ebb tidal currents are approximately equal in speed, they are collinear only across the narrowest zone, being separated as the strait enlarges. This flow separation has possibly generated two main populations of flood and ebb bedforms, which migrate at high angle and with reverse direction in the two dune-bedded zones of the strait. Moreover, because of a strong water-mass stratification due to a marked difference in salinity, the denser flood constituent dominates over the lighter ebb current. As a consequence, the largest dunes observed in both the strait opposite depositional areas have northward-oriented lee faces, whereas superimposed smaller dunes have southward-oriented fronts, reflecting flood and ebb tidal constituents, respectively. Erosional features in the central sill, uncharacteristic bedforms having orientations not explained by modern currents and coarser grains in some places implying thresholds of motion larger than the modern currents all suggest that different, most likely, larger currents, probably occurred at some point in the past. It is suggested here that these features reflect an early stage of strait connection, during the initial sea-level rise after the Last Glacial Maximum. The data reviewed in the present work summarise the complex sedimentary dynamics of the modern Messina Strait and allow a comparative analysis of the ancient strait exposed along the modern margins, also suggesting insights for other examples of tidal straits developed under similar oceanographic conditions.