Hydrodynamic controls on the distribution of surface sediments from the southeast South American continental shelf between 23°S and 38°S

Abstract

This study aims to explore the hydrodynamic factors that influence the depositional processes on the upper continental margin of southeastern South America. Mineralogy and the activities of natural (40K) and artificial (137Cs) radionuclides from surface sediment samples were used to evaluate the provenance and oceanographic controls on depositional processes. High values of the smectite/illite (S/I) ratio indicate that the Rio de la Plata (RdlP) is a major source of terrigenous sediments to the continental shelf between 38°S and 27°S and that its importance decreases towards lower latitudes. This is also corroborated by the high radionuclide 40-K activity found in muddy sediments. The kaolinite/illite (K/I) ratio highlights the interplay between two hydrodynamic controls in the sediment distribution of the study area: (i) the Brazil Current, which supplies the shelf mostly in the northern portion of the study area with kaolinite-enriched clay minerals from tropical regions, and (ii) the Brazil Coastal Current, which transports the clay minerals, mainly illite and smectite, from the RdlP to the southern portion of the study area (up to 27°S). This is also corroborated by high 137-Cs activities. In the deeper parts (>300m) of the study area, high calcite content indicates the progressive decrease in terrigenous input towards the outer shelf and upper slope.