Holocene hillslope evolution through extreme rainfall induced landslides in the mountainous region of Rio de Janeiro, Brazil: Geochronology and paleoenvironmental evidence

Abstract

Geomorphological, geochronological and paleoenvironmental data were integrated to examine how Holocene environmental changes have influenced geomorphic responses, particularly recurrent landslides and colluvial deposition on the steep slopes of the Rio de Janeiro mountains. This study focuses on a pilot hillslope (32°) with a concave-up topographic hollow and associated shallow translational landslides, enhancing our understanding of how extreme climatic events shaped landscape evolution in the region prior to significant human impact. Recently, the hillslope was reactivated by a shallow landslide triggered by the extreme rainfall event of January 2011, which resulted in over 3600 landslides in the region. Subsequent deep gullying exposed a deposit (>3 m) containing buried, organic-rich colluvial layers. Comprehensive analysis was conducted on sedimentological, geochronological (AMS ¹⁴C), palynological, taphonomic and carbon isotope (δ¹³C) data from this deposit. The geochronological results indicate that the hillslope's evolution involves recurrent shallow translational landslides throughout the Holocene, with organic colluvial layers dating from 10 148 cal years BP at the base to 663 cal years BP at the top. Charcoal fragments of various sizes suggest frequent paleofires during the Holocene. Palynological and taphonomic analyses reveal post-fire herbaceous-shrubby vegetation and ferns (Asteraceae, Poaceae, Alchornea, Baccharis, Celtis and Polypodium), with predominant pollen grains and spores showing mechanical damage indicative of high transport energy and physical stress conditions. The rarefied post-fire vegetation and pioneer forest genera identified in the deposit are associated with a hydrological and mechanical dynamics that likely created less stable conditions on the slope system, promoting the occurrence of shallow landslides throughout the Holocene.