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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° 55′S, longitude 43° 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 ≈ 5.0 kyr BP; a sediment package marked by sudden textural and compositional changes, accumulated between ≈ 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 ≈ 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 ≈ 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 ≈ 3.5 and 3.0 kyr BP, the sediment accumulation rate (SAR) was the lowest of the last ≈ 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 ≈ 3.0 and 1.9 kyr BP, associated with the highest SAR of this record, up to ≈ 27.4 m/kyr BP. Between ≈ 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 ≈ 4.5 m/kyr and, between ≈ 1.8 and 0.8 kyr BP, the study site was again under the marine influence. Foraminifera were not found, after ≈ 0.8 kyr BP, in the SP2 core. However, the occurrence of a shallow marine environment was identified in another core, between ≈ 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 ≈ 8–7.5 kyr BP may also have induced changes in the study area.

Living (stained) benthic foraminifera from the continental shelf of Santos Basin, southeastern Brazilian continental margin, were investigated to understand their distribution patterns and their relationship with chlorophyll-a and particulate organic matter contents. Sediments were collected from 44 to 203 m of water depth. This study analysis benthic foraminifera and abiotic data, including grain size, total organic matter, distributions of ocean chlorophyll-a and particulate organic matter. Foraminiferal diversity and richness were positively correlated to the chlorophyll-a and particulate organic carbon contents. Stations located on the southern portion of the study area and sampled during austral winter are probably under the seasonal influence of the Brazilian Coastal Current intrusion, and Rio de la Plata and Lagoa dos Patos plumes. Ammonia rolshauseni, Textularia? sp. and Gyroidina umbonataare present within the foraminifera assemblages at these stations. On the other hand, stations in the north, are characterized by the presence of Uvigerina peregrina, Quinqueloculina atlantica, Hanzawaia concentricaand Bulimina marginata, and they are influence of Cabo Frio’s southward-moving plume.