SCLERACTINIAN ASSEMBLAGES UNDER SEDIMENT INPUT: THEIR CHARACTERISTICS AND RELATION TO THE NUTRIENT INPUT CONCEPT

In the geologic record, scleractinian-dominated "turbid-water" buildups (TWB) that grew under terrigenous input, and neritic marls rich in corals are common. Many extant corals can cope, to varying degrees, with terrigenous turbidity and sedimentation, and acclimate to increased heterotrophy. Corals resilient to sediment input are large-polyp, massive to platy taxa, or are branched, or small-polyp forms of high physiological tolerance. In recent siliciclastic environments, the mentioned corals may comprise siltation assemblages of TWB and level-bottoms. Conversely, excess input of dissolved inorganic nutrients (DIN) into clear water readily impedes corals. Experiments show that most particulate organic matter (POM) is "treated" as sediment by corals. A large fraction of nutrient elements of POM is available to corals only after microbial processing into DIN. Beyond some threshold of sediment input and accumulation, the ecologic effects of sedimentation and soft substrate thus tend to prevail over effects of a (correspondingly lower) rise of DIN. Relative to fossil clear-water reefs, coral assemblages of fossil TWB show an increased proportion of sediment-resistant taxa. Late Jurassic to Cretaceous TWB are dominated by plocoid and/or thamnasterioid forms and, since the Early Cretaceous, by small-polyped sediment-tolerant taxa; Cainozoic assemblages consist mainly of cerioid and plocoid forms. Under increased sediment input, accumulation of discrete TWB was quenched, and level-bottoms of both corals and non-coral biota formed. In the resulting coral marls, depending on turbidity, sedimentation and substrate stability, the coral fauna is dominated by sediment-resistant colonial taxa and/or by solitary forms. In coral marl environments, because of terrigenous input, the level of POM (mainly phytoclasts) was elevated, but probably was treated largely as sediment by the corals. During scleractinian evolution, both progressive photoautotrophy and invasion of oligotrophic settings contrasts a Late Cretaceous to Cainozoic increase in the relative number of reefs in marginal-marine/ siliciclastic settings. This suggests that upon evolutionary diversification, a wider total trophic range can be covered by coral assemblages.