Influence of the Holocene relative sea level on the coastal plain of Sepetiba Bay (Southeast Brazil)

This work analyzes the results of the SP2 core (50.30 m length) collected in the coastal plain of Sepetiba Bay (SB; SE Brazil), 2 km away from the Guandu River mouth (latitude 22 degrees 55 ' S, longitude 43 degrees 46 ' W). It aims to study the influence of the Holocene relative sea level (RSL) on the coastal plain of SB. After description, the SP2 core was sampled at every 5 cm for grain size, geochemical, mineralogical and foraminiferal analyses. Four radiocarbon dates obtained in the first 18 m were modulated with Bayesian statistics. The SP2 core is composed of basement rocks, gneisses (50.30-43.80 m); river sediments (43.80-21.00 m) up to about approximate to 5.0 kyr BP; a sediment package marked by sudden textural and compositional changes, accumulated between approximate to 5.0 and 4.7 kyr BP; muddy sediments deposited between 4.7 and 0.8 kyr BP (18.80-3.35 m) with intercalations of sandy levels; an upper part of disturbed embankment sediments used for the implantation of Santa Cruz Thermoelectric Power Plant. The fluvial sands accumulated before approximate to 5.0 kyr BP were deposited before the maximum Mid-Holocene relative sea level. They were probably eroded and transported by the drainage network of the N region of SB, namely by the Guandu River. Between approximate to 4.5 and 3.5 kyr BP, the foraminiferal abundance and assemblage composition indicate that the study site was a shallow marine environment, due to the Mid-Holocene relative sea-level highstands (MHSLH). Between approximate to 3.5 and 3.0 kyr BP, the sediment accumulation rate (SAR) was the lowest of the last approximate to 5 kyr BP and the study site was exposed to subaerial weathering processes, in a scenario of the relative sea-level drop. A new phase of marine influence was recorded between approximate to 3.0 and 1.9 kyr BP, associated with the highest SAR of this record, up to approximate to 27.4 m/kyr BP. Between approximate to 1.9 and 1.8 kyr BP, the study site was under subaerial weathering processes. Since then the SAR reduced significantly to a mean value of approximate to 4.5 m/kyr and, between approximate to 1.8 and 0.8 kyr BP, the study site was again under the marine influence. Foraminifera were not found, after approximate to 0.8 kyr BP, in the SP2 core. However, the occurrence of a shallow marine environment was identified in another core, between approximate to 1.4 and 0.35 kyr BP, in the Guaratiba Mangrove, NE of SB. The results of the SP2 core suggest that MHSLH left a striking record in the study area, although some of the identified environmental changes are probably related to adjustments of the lower course of Guandu River and tidal channels and variations in the configuration of sandy littoral strands. The growth of a barrier-island system, the Marambaia barrier island, since the last approximate to 8-7.5 kyr BP may also have induced changes in the study area.