Rhodoliths are nodules mainly composed of crustose coralline algae with subordinate encrusting organisms, formed by successive overlapping encrustation. The subspheroidal rhodoliths from the Vitória-Trindade Seamount Chain (Jaseur Seamount and Trindade Island shelf; Brazil, southwestern Atlantic), sampled at water depths from 65 to 74 m, were built by crustose coralline algae (Harveylithon, Lithophyllum, Lithothamnion, Mesophyllum, Roseolithon, and Sporolithon) and subordinated encrusting foraminifera (agglutinated, unidentified hyaline, and Homotrema rubrum), bryozoans, serpulids, and balanids. Successive taphonomic phases of bioerosion, boring filling, and cement precipitation modified the original rhodolith inner structures resulting in a complex structureless mass of coralline algal fragments, encrusting organisms, borings, lithified fine-grained sediment, and carbonate cement. Borings include the ichnogenera Entobia (produced by etching sponges), Gastrochaenolites (boring bivalves) and Trypanites (polychaetes and sipunculid worms). The lithification of the material filling the borings (micrite and carbonate cements) created new substrates for subsequent bioerosion. Radiocarbon dating of selectively sampled invertebrate and algal skeletons in the rhodolith interior yielded calibrated ages of hundreds of years (up to 912 ± 152 years cal. BP on the Trindade insular shelf and up to 763 ± 131 years cal. BP on Jaseur Seamount). These values indicate growth rates from 0.1 to 0.5 mm/year, which are significantly higher than those recorded from rhodoliths at similar depths on the Brazilian shelf. Low sedimentation settings and high productivity at the tops of the seamounts and island shelf promoted the growth of nodule frame builders, both autotrophs and heterotrophs, and favored continuous activity of bioeroders.

Initiatives such as the United Nations Decade of Ocean Science for Sustainable Development and Seabed 2030 promote seabed mapping worldwide. In Brazil, especially on the Espírito Santo Continental Shelf, high-resolution seabed mapping has revealed an unknown complex seascape. Circular depressions (CDs) were mapped for the first time in the Costa das Algas Marine Protection Area. Herein, we aim to present the CD metrics characteristics and discuss their relationship with morphology and relevance as a habitat based on multibeam bathymetry and ground truthing. A total of 3660 depressions were mapped between 46 and 85 m in depth, reaching an area of 460 m² and 5 m relief. The continental shelf morphology was subdivided into three sectors based on morphology: inter-valleys, valley edges, and valley flanks, and eleven sites were selected for direct sampling/imaging at the CDs along the sectors. The direct sampling was carried out by scuba-diving with video images and sediment samples collected inside and outside the depressions. The deeper central parts of the circular depressions appear to function as a sink, presenting aggregations of rhodoliths or other carbonate fragments. In most inter-valley depressions, mounds were observed along the edges of the depression. We did not have any indication of gas seeps and no clear sedimentological or morphological control on their occurrence. We first hypothesize that their origin results from combined diachronous processes. The circular depressions mapped at high resolution could be related to sea level processes acting during the last glacial period and shelf exposure, i.e., relict features. The CDs are responsible for biomass aggregation and fish bioturbation, forming holes and rubble mounds, representing a modern process occurring on a centimetric scale. The data collected so far indicate that this fine-scale feature is an important habitat for different fish species. The modern maintenance of these structures could be due to low sedimentation regime areas shaped by biotic excavation.