Summary

A seabed sediment-sampling survey conducted on the Pico insular shelf found abundant rhodoliths between −64 and −73 m off the south coast of the island. These were small and mainly ellipsoidal in shape with a maximum diameter of 3.75 cm. Granules and small pebbles of eroded basalt were also a typical component of these samples. Thin algal crusts were secreted on basalt pebbles by the coralline red algae Phymatolithon calcareum which, in turn, were covered by Lithophyllum incrustans. Additional samples were collected by snorkelling at Maré (Lajes do Pico), a shallow lagoon (2–4 m in depth) on the south coast. Here, rhodoliths are mostly spheroidal and bigger (maximum diameter of 4.75 cm), formed by thin thalli of P. calcareum. Based on these results (distribution of shapes and species) and previous work on the characteristics of fossil specimens from Neogene deposits on Santa Maria Island (Azores) and other North Atlantic Archipelagos, an empirical depositional model is proposed for the development of rhodoliths on the Pico Island shelf: (1) Nearshore rhodoliths, formed solely by P. calcareum, are subjected to a wide range of currents and waves resulting in their spheroidal shapes. However, those of Maré lagoon are protected from offshore transport and tend to grow larger than deep-water rhodoliths; (2) Although not sampled, there must be middle shelf rhodoliths formed solely by P. calcareum that tend to form more ellipsoidal shapes due to seafloor oscillatory movements caused by waves crossing the shelf; (3) During storms, these middle shelf rhodoliths are then transported to the outer shelf, where L. incrustans overgrows the initial cover of P. calcareum. Shallow associations are normally larger, reflecting therefore, a longer life spanthan the deeper associations. Transport by storms appears to be an important factor in the formation of some deep-water rhodoliths around volcanic oceanic islands subjected to high-wave energy. However, their exposure to an energetic environment and likely frequent offshore transport does not allow them to grow as large as those from shallow-water.