Morphological characteristics of rhodolith and correlations with associated sediment in a sandstone reef: Northeast Brazil

The continental shelf of Northeast Brazil is a unique geological region, consisting of shallow-water rhodolith beds. However, very little information exists about the complexity of this part of the continental shelf and its biogenic sedimentation. The present study analysed the characteristics of rhodoliths (size, shape and structure) and compares these characteristics with the associated sediments (Grain size, biogenic composition and percentage of CaCO 3 ). The study area was located on a sandstone reef area on the inner continental shelf of Northeast Brazil. Our results show that recently dead rhodoliths are the main component of the pebble fraction with mean diameter near to 2 cm. The focal area is characterised by beds of rhodoliths, featuring individuals of all shapes (discoidal, ellipsoidal and spheroidal), with the ellipsoidal shape being the most abundant. The larger rhodoliths exhibited a near spheroidal shape, and their outer growth forms show high branching density (BIV). The associated sediment was predominantly composed of carbonate from marine fauna and flora, and calcareous algae contributed most to the sand fraction of the sediments. Foraminifera and molluscs were the most abundant biogenic fauna associated with the sediment. The grain size of the sediment was mostly medium and coarse carbonate sand. The results illustrate that there are correlations between the morphology of rhodoliths and the associated sediment. The biogenic composition of the sediment (Rhodophyta and Rizopoda) was correlated with different branching densities of rhodoliths (BII and BIV). The distribution of the sediment grain size also correlated with size, shape (diameter and sphericity) and structure (Branching BIII and BIV) of rhodoliths. Therefore, our results highlight that the correlations between associated sediment and diameter of rhodoliths is a reflection of the abiotic conditions, being principally determined by depth and hydrodynamic energy.