Evolution of reefs and reef attributes through time has long been the focus of research, with conceptual models deeply influenced by modern Caribbean coral buildups. Though many ecological conditions (e.g., wave action, light,... [ view full abstract ]
Evolution of reefs and reef attributes through time has long been the focus of research, with conceptual models deeply influenced by modern Caribbean coral buildups. Though many ecological conditions (e.g., wave action, light, nutrient flux) are recognized as influential parameters, a rationale linking these parameters genetically with reef types, and their boom and bust pattern, is still missing.
Cenozoic coral buildups are abundant and well exposed in the Mediterranean region, allowing nearly complete along-dip observations of the facies. The types of buildups are analyzed through three consecutive steps: 1) on the basis of rock volume, textures, associated sediments, and light-dependent skeletal components, as a proxy for water depth (i.e., light penetration and wave energy); 2) in the context of global environmental conditions (e.g., temperature gradients and carbon cycling as indicated by oxygen and carbon isotope records); and 3) through processes that promoted the waxing and waning of the reef limestones (e.g., relief functionality, nutrient flux and trophic requirements).
Danian to early Ypresian buildups, without a significant break in shallow tropical carbonate sedimentation after the Cretaceous-Paleogene boundary, attest to the rise of modern coralgal communities and the first evolutionary steps of the main lineages of larger benthic foraminifers. The warm Eocene, when temperature gradients were minima, was dominated by LBF, though corals were diverse and occurred in small biostromes and buildups in the mesophotic zone. Coral buildups re-emerged during the Late Eocene cooling; the subsequent Oligocene cooling coincided with the first diversification of the zooxanthellae and coral reorganization, and increase in coral accretion potential. The subsequent late Oligocene-Miocene warming saw the diversification of other groups of LBF, and coralline red algae diversified; corals were subordinated and, along with red algae, formed abundant buildups. Paralleling the second step in zooxanthellae diversification and the progressive increase in global temperature gradients, the Late Miocene saw the onset of modern reefs occupying the shallow euphotic zone and forming a rigid framework up to sea level. The upward migration of the corals is seen as a response to the strengthening of the thermocline.
These trends show that larger coral buildups coincide with strong temperature gradients. Coral reorganization and changes in the building capacity concur with global cooling episodes and diversification of zooxanthellae. In contrast, LBF became the main actors associated with weaker thermal gradients and reduced water-mass stratification, likely resulting in reduced turbulence.
