Vertical zonation of bioturbation and mass movements in the Portimão Bank (Gulf of Cadiz, SW Iberia)

Abstract

This work presents results from bioturbation intensity and trace fossil analysis of the piston core PC07, recovered at Portimão Bank during the MONTERA cruise in 2012. The aim was to investigate the potential of bioturbation analysis as an indicator for mass movement events. Bioturbation and trace fossil identification was complemented with results from sedimentological analysis (grain-size, carbonates, organic matter), measurements of magnetic fabric parameters (mass magnetic susceptibility and anisotropy of magnetic susceptibility), and AMS 14C dating. Considering the vertical zonation of bioturbation, magnetic fabric changes, sedimentology and AMS radiocarbon dating, two intervals were identified, Interval A (0-180 cm) and Interval B (180-356 cm), being the limit between them placed at 180-181cm. These intervals seem to replicate almost the same vertical zonation of bioturbation and the trend of carbonates and organic matter changes downcore. The magnetic fabric parameters (degree of anisotropy – Pj and shape – T of the magnetic susceptibility ellipsoid) show the major change at about 180 cm. Geochronological results indicate that the age of sediments in Interval A is 8590 yr BP (31-32 cm) and 14 300 yr BP (103-104 cm). Otherwise, ages in Interval B are the following: 5035 yr BP (181-182 cm), 8900±50 yr BP (219-220 cm), 18999±120 yr BP (303-305 cm). These results point to the emplacement of sediments of Interval A on top of youngest sediments of Interval due to landsliding. Since no internal deformation is seen, probably this indicates translational movement and short transport from the source.