Marcadores orgânicos na interface continente-oceano: história deposicional recente na plataforma continental brasileira entre 10ºS e 23ºS

Abstract

Rivers are natural receptors of all kinds of substances from human activity, consisting of an important transport route of terrigenous materials to the coastal zone. Despite the effects of diagenesis on organic matter, the sediments deposited in the region of its mouth can be used as tools of geochemical recording, in order to determine the depositional history of a given basin. The objective of this study was investigate, using organic markers preserved in sediment cores, the transfer of material through the continent-ocean interface in the zones influenced by Doce, Contas and São Francisco rivers, in order to infer the main sources of natural and anthropogenic inputs to these regions from a historical perspective. This study was carried out with the evaluation of cores recovered in the continental shelf, associated with each river mouth regions, compared with a core recovered from the Cabo Frio upwelling region, without any significant fluvial influence. Evaluation of the samples was done through a multi-analytical approach, involving the analysis of carbon and nitrogen stable isotopes, total organic carbon, hydrocarbons (aliphatic and aromatic) and steroids in order to infer the natural and anthropogenic influence in each region in a decadal scale. The results indicate that off the mouth of the Doce River occurs active sediment deposition, following the raining seasons in the region of its basin, with continental input of natural and anthropogenic material. On the other hand, the evidence obtained in the regions off the mouths of Contas and São Francisco rivers indicates the occurrence of lower continental inputs, as a result of the climate/size of the basin as well as the constructions along the river bed (case of the dams along São Francisco River), which determine lower input of terrigenous material to the continental shelf and, therefore, make it difficult to establish a temporal correlation with eventual anthropogenic contributions in its basins. The core obtained off Cabo Frio upwelling region, similarly with Contas and São Francisco cores, didn’t indicate occurrence of significant anthropogenic contributions. Although this core was obtained in an unfavorable area for organic compounds, it was possible to identify periods of more effective SACW intrusions over continental shelf area, using nalkanes and steroids results.