Scleractinian assemblages under sediment input: their characteristics and relation to the nutrient input concept

In the geological record, scleractinian-dominated turbid-water bioconstructions that accumulated under substantial terrigenous input, along with shallow neritic marls rich in well-preserved corals are common. These occurrences indicate that beyond some threshold, the ecologic and taphonomic effects of sedimentation prevail over the effects of nutrification. Field data and experiments on recent corals show that some taxa cope well with terrigenous turbidity and sedimentation, and acclimate to increased heterotrophy. Colonial corals resilient to sediment input commonly are massive to platy forms of high skeletal plasticity and with large polypars, to effectively reject sediment, or are branched species providing a very small sedimentation area cleaned by feeble currents, or small-polyped massive taxa that tolerate intermittent veneering by sediment. These corals may comprise siltation assemblages of recent turbid-water reefs and level-bottoms, in siliciclastic environments. Conversely, excess input of even low amounts of dissolved inorganic nutrients into clear waters adversely affects many coral species. Fossil turbid-water bioconstructions (TWB) were situated in inner shelf to shore zone settings or near storm wave base on shelves or ramps, did not stack into reef complexes, mostly show a cluster to segment fabric, are scarce in or devoid of frame pores with marine cement, and lacked a high carbonate slope. Relative to fossil clear-water buildups preserved in pure limestone successions, fossil TWB contain (moderately) diverse coral assemblages, with an increased proportion of sediment-resistant forms. Aside from coral behaviour that is elusive in fossils, the sediment resistance of corals was mainly determined by colony shape and polypar size, but small-polyped, sediment-tolerant corals became common since the Early Cretaceous. Late Jurassic to Cretaceous turbid-water coral assemblages are dominated by plocoid or thamnasterioid forms; corresponding Cainozoic assemblages consist mainly of cerioid and plocoid taxa. In fossil TWB, bioerosion and encrustation are similar or higher than in contemporaneous clear-water buildups. Under higher sediment input, accumulation of discrete TWB was quenched, and level-bottoms of both corals and soft-substrate biota formed. In the resulting coral marls, depending on turbidity, sedimentation and substrate stability, the coral fauna consists largely of both solitary corals and (mainly) sediment-resistant pseudocolonial/colonial forms, or is dominated by solitary corals. Many corals show growth anomalies resulting from partial mortality due to episodic, rapid sedimentation and unstable substrate. In coral marl environments, because of terrigenous input, nutrient levels probably were elevated, yet macroboring and encrustation are very scarce. Beyond some threshold of terrigenous sediment input and accumulation, the ecologic and taphonomic effects of sedimentation and unstable substrate prevail over the effects of nutrient enrichment. During the Meso-Cainozoic evolution of scleractinians, increasing photoautotrophy and progressive invasion of oligotrophic environments is in contrast to a Late Cretaceous to Cainozoic increase in the relative number of reefs in marginal-marine/siliciclastic settings. This trend may be explained by second-order sea-level fall and shrinking of epicontinental carbonate seas and isolated platforms, driving corals to more narrow, attached shelves, and by a wider total trophic range in which distinct coral assemblages can thrive, as a result of coral evolution and diversification. (C) 2004 Elsevier B.V. All rights reserved.