Responses of foraminiferal assemblages to hydrodynamics and sedimentary processes on tropical coastal beachrocks

Abstract

Coastal environments are very influenced by hydrodynamics and associated sedimentary processes. The assemblages of benthic foraminifera are structured by environmental characteristics, given the sensitivity and specificity of living populations or the presence of dead specimens, making them good indicators of the dominant processes in coastal environments. The present study used the benthic foraminifera as a proxy to assess environmental characteristics in beachrocks of Brazilian tropical sandy beaches. Samples were collected at the base of coastal beachrocks on the beaches Pina, Enseada and Toquinho (northeastern, Brazil) to describe foraminiferal assemblages (27 samples), and to grain size analysis and CaCO3 content determination (27 samples). The sediments were predominantly composed of moderately sorted and approximately symmetrical medium to coarse sand of terrestrial origin. The littoral drift, rip currents, and waves are the main factors driving the size, sorting and redistribution of the sediments. A total 6829 foraminifera were identified belonging to 42 genera and 76 species. The assemblages were dominated by Amphistegina lessonii, Archaias angulatus, Massilina pernambucensis, Quinqueloculina lamarckiana, Textularia agglutinans and Triloculina laevigata, and were mostly composed of relict tests probably originated and deposited during the last marine static period of sea level. The structure of foraminiferal assemblages significantly varied among beaches and was related to the dominant hydrodynamics and sedimentary processes. Higher richness and abundance of foraminifera occurred in areas with lower hydrodynamics and finer sediments, like Pina, whereas more intense hydrodynamics tended to prevent the deposition of fine sediments and foraminifera, and cause greater degradation of the tests, as observed in Enseada. These fidings highlight the suitability of using foraminiferal assemblages to understand the dominant environmental processes in coastal areas dominated by beachrocks.