Mediterranean bottom current deposits: an example from the Southwestern Adriatic Margin

The identification of bottom-current deposits is a key to understanding the long-termdeep-sea circulation and its changes through geological times. The Southwestern AdriaticMargin (SAM) is a small Mediterranean sub-basin that represents a key site to study bottomcurrentdeposits in a Mediterranean context and hence to improve our knowledge of changes inMediterranean deep-water circulation during the recent geological past. The SAM is characterizedby complex stratification and circulation related to an interaction between two south-flowingbottom water masses: the cold North Adriatic Dense Water (NAdDW), formed in the shallownorthern Adriatic through cold wind forcing and winter heat loss, and the highly saline LevantineIntermediate Water (LIW), generated in the Eastern Mediterranean through intense evaporationand flowing along the slope in a depth range of 200-600 m. Chirp-sonar profiles, TOBImosaics and sediment cores acquired along the SAM reveal distinctive sediment drift types(elongated, plastered and isolated drifts) and extensive fields of sediment waves. Non-depositionaland erosional features related to bottom-current activity include moats between drifts and the steepslope, widespread upper-slope erosional areas and extensive furrowed areas, which are particularlydeveloped where change in slope orientation blocks the current circulation. The distribution,morphology and size of bottom-current features along the SAM result from an interaction betweencurrent regime and slope morphology, characterized by structural highs perpendicular to the slopecontour (e.g. Dauno Seamount), multiple slope incisions (e.g. Bail Canyon and slump scars) andextensive block-slide deposits. Morphobathymetric and seismic stratigraphic data on the SAMshow that bottom-current deposits are best developed where the regional slope flattens seawardof a very steep, often erosional, upper slope. The roughness of the lower slope, in particular,seems to correlate with the complexity and decreasing size of the bottom-current deposits.Like other land-locked basins, the Adriatic underwent dramatic palaeogeographical and palaeoceanographicrearrangements during the Late Quaternary sea-level oscillations. Indeed, duringthe Last Glacial Maximum (LGM), most of the areas where NAdDW is formed today weresubaerially exposed. Concurrently, during glacial times the LIW production was probablyreduced compared with the present-day conditions. The SAM slope is a key site to study theimpact of changing current regime on late Quaternary slope deposits. Other Mediterranean lateQuaternary contourite deposits are either in water depths compatible with the LIW, particularlyin the case of shallow sill basins (e.g. Sicily, Corsica Channel), or at the slope base reflectingthe flow of Mediterranean deep waters. The SAM bottom-current deposits, instead, seems torecord the changing interaction between two distinctive bottom-hugging currents along thesame pathway.