Halimeda bioherms

Abstract

Halimeda bioherms have been described from three parts of the Great Barrier Reef: from the Sahul Shelf, from the Makassar Straits, and from the Nicaraguan Bank. They form mounds that are up to 20 m height and are comprised of the skeletal remains of various species of Halimeda (H. opuntia, v. hederacea, and H. copiosa with subordinate amounts of H. fragilis, H. discoidea, and H. gracilis) together with large benthic foraminifera, mollusks, and bryozoans. The bioherms are bedded, sometimes transparent and often multigenerational, and sit astride the late Pleistocene transgressed surface. They have grown at the same time as the adjacent reef ecosystem and at rates almost equivalent to the reef systems. This suggests not only a critical relevance of calcareous algae as important additional CO2 sinks but when estimated on a global basis it also doubles the contribution of shallow water sediments (in conjunction with reefs) as CO2 reservoirs.