Highly bioeroded rhodoliths from the Vitória-Trindade seamount chain (Brazil, southwestern Atlantic)

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.