Short-term sediment dynamics on a meso-scale contourite drift (off NW Iberia): Impacts of multi-scale oceanographic processes deduced from the analysis of mooring data and numerical modelling

Abstract

Possible driving mechanisms of sediment dynamics in a contourite drift–moat–obstacle system on the continental slope off the NW Iberian Peninsula are investigated in this study based on a statistical analysis of an 18-month long mooring record in combination with a 3-dimensional process-based numerical modelling. Analysis of the mooring data indicates that the modern hydrodynamic regime in the study area is non-linear, with a significant impact of internal tides and a non-negligible influence by the Earth's rotation. A consistent counteraction between barotropic and baroclinic tidal currents, mainly at M2 frequency, results in a weak bottom flow that rarely exceeds the threshold velocity for resuspension of sandy silts in the system. This observation explains a Late Holocene hemipelagic (vertical settling) deposition of a fine-grained (8–10μm) sediment surface layer over the entire drift as well as inside the moat. In order to unravel possible driving forces for active sediment remobilization during the Deglacial/Early Holocene interval between 17,000 and 4000calyr BP that is consistent with the sedimentary record, different oceanographic scenarios were designed for a numerical study. Simulation results demonstrate that pulse-like oceanic density fronts and associated km-scale eddies, which originate from instability in the transition zone of two water masses (Labrador Sea Water, Mediterranean Outflow Water), being aided by tidal and geostrophic currents and locally strengthened by topography, are the most reasonable mechanisms inducing contouritic sediment transport and deposition that are observed in the sediment-core transect. These density fronts are of subordinate impact for the modern contourite drift which is bathed by Labrador Sea Water nowadays. However, a vertical downward migration or expansion of the overlying Mediterranean Outflow Water by ~300m between 17,000 and 4000calyr BP would facilitate a generation of significant density fronts at the depth of the contourite drift. This MOW migration is believed to leave imprints on other contourite systems located at similar depth in the region.