Submerged Pleistocene spodic horizon remnant exposed on the inner continental shelf off Guanabara Bay (Rio de Janeiro, Brazil)

摘要

A deposit dated from 40 cal ka bp is interpreted as a remnant of a spodic B horizon exposed on the seabed adjacent to Guanabara Bay at an average depth of 22 m. Bathymetric data, side scan sonar imagery, and underwater images were used to map the expressive feature, never informed before. It is a semi-consolidated muddy-sand deposit with an escarpment up to 4 m and 2.5 km in length (E-W) and 1.5 km in width (N-S). Grain size and geochemical analyses were carried out, and the hypothesis of being formed by podzolization was confirmed. The paleo sea level at the time of the pedogenesis was estimated to be above the maximum height of the global eustatic curves related to Marine Isotopic Stage 3. The feature exhibits a quartzose-sand constitution, defined as an erosion surface exposed at ca. 12 ka bp. The coastal retrogradation of upper unconsolidated sediment during the last transgression contributed to the settlement of a sandbank at the bay mouth.