Characterization of the continental shelf and slope of the State of Bahia, from the cluster analysis of sedimentological data and the foraminifera microfauna

Abstract

This study aimed to characterize the continental shelf and slope of the State of Bahia, based on the multivariate analysis applied to sediments data and the foraminifers’ microfauna. Thirty-eight samples from the bottom sediment surface were analyzed, which were processed in accordance with the standard methodology for sediment studies (grain size and composition) and microfaunal analysis. From the sediment fraction used for the foraminifers’ micro fauna analysis, the first 300 tests were randomly sorted and identified. For the multivariate analysis, it was used species that have at the same time frequency of occurrence higher than 50% (constant) and relative frequency greater than 5% (major) in, at least, two samples. The methodology consisted in the application of the technique of multivariate grouping in which it was set the squared Euclidean distance defined as a metric formation of groups, applying the methods Ward’s and k-Means. A cluster analysis of species (R mode) was performed based on the absolute frequency data for each different bathymetric range. Then, a cluster analysis was applied between the samples (Q mode) based on the percent of sediment present in each sample. The dendograms data integration, generated by the multivariate analysis, revealed that siliciclastic sands predominate in the inner and middle continental shelves, carbonate gravelly sands dominated in the outer shelf and carbonate muddy sands were the major sediment type found in the continental slope. The distribution of Archaias angulatus and Peneroplis proteus was closely related to the occurrence of siliciclastic sands, while the Discorbis mira was associated with carbonate sands, and Cassidulina laevigata and Cibicides pseudoungerianus were associated to carbonate muddy sands. The distribution of Amphistegina lessonii, Amphistegina gibbosa and especially Peneroplis carinatus was associated with the sediments from the transition between siliciclastic and carbonate sands. The planktonic Globigerinoide ruber was particularly found in carbonate muddy sands located farther from the coast.