A new reef classification model with insights into Phanerozoic evolution of reef ecosystems

Abstract

AbstractOrganic reefs are largely calcareous build‐ups of sessile organisms, commonly in shallow marine ecosystems, with a long Precambrian to Modern history. Despite a long history of study, the reef classification is still open to development due to reefs encompassing diverse organic and inorganic components with various imprecise and synonymous terms. In this study, a new classification model for organic reefs is expressed in two hierarchical levels.Level 1: Reef outline geometry has two major forms: bioherms (prominent relief and laterally constrained) and biostromes (low relief and large lateral extension interbedded with non‐reef sediments), within which constructional components highly vary.Level 2: Four reef constructor groups form end‐members in a pyramid diagram: macroskeleton (S), microbia (Mi), matrix (Ma) and cement (C). Thus five reef types are recognized depending on the composition and structure of these constructors: (i) S‐supported reef; (ii) Mi‐supported reef (calcimicrobial and agglutinated microbial); (iii) Ma‐supported reef (cluster and carbonate mud mound); (iv) C‐supported reef; and (v) hybrid component‐supported reef (comprising two or more major types of reef components). This arrangement makes a more streamlined and accessible reef classification than prior schemes. Two associated non‐reef habitats that may grade into reefs are relevant: debris biostromes/banks and level‐bottom communities containing taxa found in adjacent reefs. Based on this ‘SMiMaC’ model, the structural evolution of reef ecosystems is demonstrated for the Phanerozoic, characterized by S‐supported and hybrid‐supported, and Mi‐supported and Ma‐supported structures under cooling and intense warming climates, respectively.