Coralline algae and bryozoans as major carbonate sediment components of shallow and mesophotic reefs on the Abrolhos Bank, Brazil, SW Atlantic

Abstract

The Abrolhos Bank is the largest reef complex in the South Atlantic, harboring unique pinnacle formations (“chapeirões”) and the world's most extensive rhodolith bed. Building organisms (e.g., bryozoans, crustose coralline algae, and corals) interplay with bioeroding groups (e.g., sponges, fishes, echinoderms, and polychaetes) in the carbonate production and erosion processes controlling the reef structure. Many physical, chemical, and biological erosion processes break down these calcareous organisms in several sediment grain sizes. The present study aims to characterize the superficial sediments of three Abrolhos Bank carbonate reefs with increasing depth (6–33 m) and distance from the coast (14–75 km). Surface sediments (∼5 cm surface layer) were analyzed to determine their biological source, mineralogical compositions, and grain size distribution. In the site closer to the coast (Pedra de Leste), values of organic matter and finer terrigenous sediments were higher, while offshore sites (Parcel dos Abrolhos and California Reef) were composed mainly of carbonate bioclastics. The analyzed fragments were composed of 11 large groups, comprising eight invertebrates: Annelida, Ascidiacea, Bryozoa, Cnidaria, Crustacea, Echinodermata, Mollusca, and Porifera, plus Foraminifera, Osteichthyes, and one macroalgae(Rhodophyta). Within these large groups, a total of 61 taxa were identified, including five new records for the Abrolhos Bank (two Bryozoa and three Mollusca). In shallower reefs (Pedra de Leste and Parcel dos Abrolhos), the main bioclastic component was crustose coralline algae, CCA (45 and 49%, respectively), while in the mesophotic reef (California Reef) it was Bryozoa (42%). The sponge chips' contribution to the sediment composition varied from 0.3% to 3% in Pedra de Leste and Parcel dos Abrolhos, respectively. However, it could be underestimated due to the formation of peloids or fine sediment aggregates, which mask the visualization of some fine-grained particles. The main results from sediment analyses highlight the importance of calcifying organisms (CCA and Bryozoa) and bioeroding ones (sponges) to sediment formation on different cross-shelf reefs on the Abrolhos Bank.