Post-LGM multi-proxy sedimentary record of bottom-current variability and downslope sedimentary processes in a contourite drift of the Gela Basin (Strait of Sicily)

Depositional systems accumulating under the combined influence of along-slope currents and downslope sediment-gravity flows are frequent in several continental margins. Despite being well-documented in terms of how these opposing depositional processes shape the margin architecture, many aspects related to their interaction and distinctive characters of resulting sediment accumulations remain elusive. Here we focus on the Gela Basin, the foredeep of the Maghrebian fold-and-thrust belt in the Strait of Sicily, where the Levantine Intermediate Water (LIW) and the Modified Atlantic Water (MAW) are confined by margin morphology, which amplify their velocities. Two sediment cores located on the upper and lower slope of the Gela Basin document the overlapping of along- and downslope processes since the very last phase of the Last Glacial Maximum (LGM). Detailed analysis of several proxies including sedimentary structures, sortable silt, geochemical elemental composition, oxygen and carbon isotopes, ichnofacies and foraminifera assemblages helped to disentangle the sedimentary imprints of contourites (including variations in bottom-current velocity through time) and downslope gravity-driven processes (turbidity currents and mass-transport processes). The slope experienced exceptionally high sedimentation rates up to 1300 cm kyr−1 during the last phase of LGM and early phase of Heinrich Stadial 1 (HS1), which rapidly decreased before Heinrich Event 1 (< 100 cm kyr−1). The high accumulation rates were driven by a combination of sediment input from the inner and mid shelf and lateral advection promoted by strong bottom-currents under the action of the LIW. An abrupt and brief intensification (+ 7.8 cm−1) of the MAW speed during the early phase of HS1 affected the stability of the sediment drifts, which were growing since the previous interglacial and contributed to the emplacement of mass-transport deposits around 17 kyr BP. During the post-glacial sea-level rise pulses, before and during the Heinrich Event 1, the two sediment cores registered contrasting bottom-current velocities, suggesting a progressive shoaling of the LIW and modifications at the interface between the LIW and the MAW, in response to the increased fresh water discharge from the Atlantic. Our findings suggest that sea-level fluctuations can change the thickness and core-depth of the Central Mediterranean water masses, leading to intervals of enhanced bottom-current erosion and margin instability along the outer shelf and upper slope.