Coastal dunes with high content of rhodolith (coralline red algae) bioclasts: Pleistocene formations on Maio and São Nicolau in the Cape Verde archipelago

Abstract

Abstract Rhodoliths are spherical growths (coralline red algae) that contribute bioclasts to coastal dunes in the Gulf of California (Mexico) and the Canary Islands (North Atlantic). Pleistocene dunes on Maio and Sao Nicolau islands in the Cape Verde archipelago were studied to quantify rhodolith contribution relative to other sources. Near Pilao Cao on Maio, a transverse dune at Lomba Greija covers 0.3 km 2 , exposing stoss slopes that dip 8°–10° NE and leeward slip faces that dip 28°–32° SW and SE. Point counts on thin-section samples show that basalt and other non-carbonate materials account for 5%, on average, whereas fine matrix and voided space (dissolved grains) account for 67%. Among remaining identifiable bioclasts (coralline red algae, mollusks, corals, foraminifera, and echinoderms), rhodolith grains with an average diameter of 0.5 mm account for 74%. Near Carrical at Covoadinha de Chacina on the SE coast of Sao Nicolau, the stoss slope dips 8° SE for 70–80 m on narrow longitudinal dunes. Point counts on a thin-section sample taken 2.5 m above basement rock, reveal that basalt and other non-carbonate materials account for 10%, on average, whereas fine matrix and voided spaces account for 60%. Among identifiable bioclasts from the remainder (coralline red algae, mollusks, echinoderms, and bryozoa), rhodolith grains ranging in size from 0.5 to 1 mm account for 96%. Potential enrichment from coralline red algae may be overlooked in coastal dunes, because content normally is described as dominated by mollusk shells, the tests from abundant foraminifera, and/or ooids.