Interpretações paleoambientais do Holoceno Tardio com base em foraminíferos e bivalves no estuário do Rio Real (Bahia-Sergipe)

Abstract

In the paper on intra-annual variation of foraminifera, the species Trochammina inflata, together with data from other authors, represents the most predominant species in both seasonal campaigns, showing that the waters of the Real estuary are oligohaline, that is, typical of environments with strong river discharges. The second article, presents paleoenvironmental interpretations showing a view of variations in estuarine environmental conditions and subtle evidence of probable sea level change (MSL) events during the Late Holocene, supported by models based on works about morphodynamic changes and the paleoecology of bioindicators paleoenvironmental, which are based on current and past conditions of the environment. In the second article, the genus Trochammina also appears as dominant, confirming the conditions of a fluvial environment evidenced in the first article on intra-annual variation. The biozones identified in the article that contemplates paleoenvironmental interpretations were identified as follows: In T01 the facies-G1(base to the 105 cm interval) and marks the existence of the Middle Estuary. Additionally, the facies-G1 and G3 (intervals 70 to 50 cm) due to being predominantly composed of a sandy-muddy sedimentary package, indicates a paleoenvironment controlled by freshwater input, typical of a fluvial environment. The G4-facies (intervals 50 to 15 cm) recorded the mollusc species Anadara sp., Anomalocardia brasiliana, Corbula caribaea, C. cubaniana, C. subrostrata, Chione cancellata, and Luanarca ovalis, evidencing the occurrence of a Low Estuary, or that is, an environment with marine influence. Facies-G2 (intervals 105 to 70 cm) and G4 registered a domain of eurihaline bivalve species associated with sandy-muddy granulometry, possibly indicating a transgressive deposit and an offshore transition. Facie-G5 (intervals 15 at the top) presents a set of brackish agglutinating species, shallow mixohalines and calcareous marine species, suggesting this moment as the current estuary, marked by shallow transgressive marine deposits. The T02 biozones, identified as: facies-Z1 (base to 175 cm) and Z3 (intervals 175 to 15 cm) present a sterile environment in bivalve and foraminifera, which suggests the presence of an environment of freshwater, and that, probably, an event occurred of regression. Facies-Z2 (intervals 205 to 175 cm) recorded the marine bivalve Diplodonta cf. notata associated with the sandy granulometry of the estuary of the river Real, suggests a shallow marine environment. Facies-Z4 (interval 15 cm at the top) represents the current estuary with marine dominance in relation to the fluvial, corroborated by the predominance of species of coastal, marine and continental shelf foraminifera transported into the estuary, for example: Ammonia beccarii, Cibicides lobatulus and Haynesina germânica. The paleoenvironmental interpretations carried out based on the analysis of granulometric and biotic data (foraminifera and bivalves) corroborate with the observations of the article on the intra-annual variation in the estuary of the river Real. The estuary exhibits a moderate hydrodynamic energy, supported by the predominantly sandy-muddy granulometry and the presence of shell fragments and autochthonous organisms. It is concluded that the present research contributed to the spatial and temporal understanding of the palaeoenvironment of the river Real estuary during the Late Holocene based on foraminifera and bivalves.