Resumo:
Environmental magnetism evidences for longshore drift distribution of Fe bearing minerals between the Paraíba do Sul and Itabapoana rivers, state of Rio de Janeiro (Brazil) Over the last decades, studies based on sedimentary provenance and distribution by longshore transport drift have been carried out in the northern coast of state of Rio de Janeiro (Brazil). Recent works focused on heavy minerals as tracers to determine sedimentary sources and transport pathways indicated a mineral distribution according to the northward and southward coastal transport cells, as well as evidences for heavy mineral transportation by the Paraíba do Sul river. Nevertheless, such works did not provided investigations on the opaque fraction — e.g., iron oxides and oxyhydroxides —, that are essential part of heavy mineral’s assemblages. Therefore, we aimed in this work: (1) to characterize the spatial variation of the magnetic properties of heavy mineral contents throughout a coastal area restricted by the Itabapoana and Paraíba do Sul river mouths (state of Rio de Janeiro); (2) to provide subsides on the iron bearing mineral provenance; (3) to evaluate the contribution of coastal dynamics to the mineral distribution along this coastal study area. Nearby the Paraíba do Sul river, where low coercivity magnetization and high magnetic susceptibilities prevailed, a bimodal pattern was observed for the remanence carriers – identified as magnetite and hematite. For the northernmost section, a higher coercivity mineral phase, which as interpreted as goethite, exhibited a gradual increase in concentration towards the central part of the study area. It was possible to verify three different sampling groups in accordance to its magnetic properties: (1) “southernmost” and (2) “northernmost” groups – respectively suggestive for a mineral provenance related to the Paraíba do Sul and Itabapoana rivers as possible major sources; and, (3) at the intermediate section of the area, possibly linked to the erosion of the Barreiras Formation bluffs. Regarding similarities and/or gradual variations in the sediment magnetic properties throughout the area, our results indicated a clear correspondence of the magnetic parameter variability with the main directions of the coastal transport cells.
