Benthic foraminifers from Holocene subaqueous deltas of the Western Mediterranean: Stratigraphic implications and palaeoenvironmental significance of the biofacies

Abstract

To better explore the potential of Holocene subaqueous deltas as sedimentary archives, we quantitatively examined the benthic foraminiferal content of the muddy wedges belonging to three major Western Mediterranean deltas: Po Delta (N Adriatic Sea), Arno Delta (Ligurian Sea) and Rhône Delta (Gulf of Lions). The application of multivariate analyses (R-mode cluster, nMDS) on a regional dataset allowed defining six Mediterranean assemblages of taxa whose distribution, through time and space, is guided by the distance from the feeding mouths and by the balance of power between river and sea processes at the sea floor. At each site, the stratigraphic distribution of multivariate faunal data efficiently tracks the main changes in riverine fluxes and hydrodynamic levels, leading to a robust evaluation of (dis-)similarities in delta growth patterns. The dominant role of fluvial processes invariably emerges from the Po record, pointing to a river-dominated regime; whereas the foraminiferal fauna in the Arno and Rhône deltas indicates a wave-dominated regime with an intense redistribution of the river load by wind-driven currents throughout the Holocene. The distribution patterns of benthic foraminiferal assemblages in the multivariate space allow the identification of six biofacies, reflecting specific subaqueous environments that typify the Mediterranean shelves under different delta regimes: i) inner shelf; ii) deep-water mud belt; iii) shallow-water mud belt; iv) distal, river-dominated prodelta; v) distal, wave-dominated prodelta and vi) proximal prodelta. These biofacies can be a useful palaeoenvironmental tool for the interpretation of ancient, shallow-marine muddy successions belonging to the western Mediterranean stratigraphic record. The stratigraphic-based multivariate approach applied in this work can strongly support the analysis of subaqueous delta successions, developed even under different delta regimes and difficult to unravel from sedimentological features alone.