Insights on the marine sedimentation of the continental shelf and upper slope off SE Brazil during the 20th century with natural radionuclides

Abstract

This paper aims to improve the understanding of the marine sedimentation of the continental shelf and upper slope off southeast (SE) Brazil during the 20th century through the study of the deposition of natural radionuclides and its relationships with other sedimentological variables in a large set of sediment cores obtained during several oceanographic cruises. The main results of this study were the determination of sedimentation rates (0.49–3.73 cm yr−1) and its geographic distribution in the area. Together with 226Ra, mud, and carbonate content data, a picture emerged of a shelf and upper slope mostly dominated by lithoclastic sediments with a spatially heterogeneous grain size distribution, inputs of allochthonous sediments, and the presence of mid-outer shelf mud depocenters. Based on the age reconstruction of the cores, it was established that the area underwent no significant variations in sedimentation patterns during the period sampled by the cores, which corresponded to the last century. Nevertheless, there were clear geographical differences within the region; thus, the area was divided into sectors suggested by the assessed variables. The southern sector is marked by the presence of mud depocenters with high sedimentation rates and 226Ra levels connected to the Brazil Coastal Current (BCC) flow. The northern sector presents irregularly distributed sediments and modern contribution from processes associated with the Brazil Current (BC) and the exchange of materials with nearby coastal systems. The central sector functions as an interface between the processes observed in the southern and both sectors. The results highlight the need to reassess the prevalence of the reworking of Pleistocene sediments and the lack of current sediment sources in the area.