Evolução paleoambiental holocênica da Baía de Sepetiba, RJ

Abstract

The Sepetiba Bay is placed in the southeastern region of Rio de Janeiro State and its evolution is strongly related to the Quaternary sea level variations, especially during the Holocene (11,000 years cal. AP present), similarly as occurred in other coastal systems. The present work aims to identify in the region the transition from a continental environment, related to the glaciers expansion, to a marine environment, which started after the sea level reached its maximum at ≈ 7,000 years AP. For this propose, sedimentological, including granulometry, geochemical (TOC, total sulfur and carbonate), mineralogical (fine fraction) and faunistical (benthic foraminifera) data were acquired. These data were obtained in three cores, named: SPT-Furnas (50.00 m of extension, next to the São Francisco Channel), T4 (286 cm of extension, in the inner zone of Sepetiba Bay) and T1 (480 cm of extension, in the Guaratiba Mangrove), all of them statistically treated by multivariate analysis. Nine 14C dating were performed for geochronological placing of the described events. The results reveal a sedimentological discontinuity of ≈ 7,000 years cal. AP in the SPT-Furnas core which separates two distinct sedimentary packages: the bottom is composed by fluvial sands deposited under high energy conditions, and the top consists of muddy silt sediments deposited under calm and high sea-level, intercalated by periods of higher hydrodynamism. Since ≈5,000 years cal. AP, periods of greater marine influence were evidenced by typical bottom bay-related foraminiferal fauna such as Ammonia tepida, Ammonia parkinsoniana, Crobroelphidium excavatum, Pararotalia cananeiaensis, Elphidium spp and Buliminella elegantissima. These episodes occurred between ≈5,000-4,000 years cal. AP, ≈3,000-2,100 years cal. AP e ≈2,000-500 years cal. AP. There were episodes of lower marine influence: ≈4,000-3,000 years cal. AP and ≈2,000-1,500 years cal AP, the last one related to the Marambaia Restinga rupture. The growing confinement was observed in the three cores and may have been responsible for: (i) the absence of foraminiferal fauna in the inner zone of the Sepetiba Bay in the last ≈500 years cal. AP, and (ii) the emplacement of the Guaratiba Mangrove in the last ≈300 years cal. AP, evidenced by a exclusively agglutinant fauna associated with high TOC and carbonate values. The integration of the data obtained in the three cores allowed to identify a sequence of events for the last ≈7,000 years cal. AP in the Sepetiba Bay: (i) a period of transition between continental and marine environment (≈7,000 6,000 years cal. AP); (ii) the beginning of oceanic influence since ≈5.000 years cal. AP, with a growing degree of confinement since 3,000 years cal. AP (after the closure of the Marambaia Sand Ridge) and greater hydrodynamism; and (iii) the establishment of the Guaratiba Mangrove, related to a higher degree of confinement of the coastal system.