Abstract:
Coral reefs are commonly considered to have developed in tropical-subtropical shallow-water regions with typical carbonate depositional environments. However, discoveries of deep-water coral reefs in the last decade have overthrown this traditional overview. The deep-water reefs are ubiquitous at least in the NE Atlantic, and distributed in all major oceans. Cold-water corals, the major constructors of the deep-water reefs, differ from tropical-subtropical reef-building corals, in terms of their heterotrophic habitat independent of light. They prefer to settle on hard and stable substrates, such as boulders and seamounts, and construct conic carbonate bodies under the oceanic conditions of high nutrient content and low sediment influx. The Porcupine Seabight on Ireland offshore is one of the most intensively studied provinces. There, thousands of the Pleistocene-recent coral reefs occur as carbonate bodies up to 2 km in width and 200 m in thickness. Depth of dense reef development ranges from 600 to 900 m, where the bottom current is enhanced by the internal tidal wave at the boundary between the Mediterranean Outflow Water (MOW) and Eastern North Atlantic Water (ENAW). Sediments recovered in IODP Expedition 307 revealed that a reef body consists of fragments of Lopheria pertusa enclosed in fine-grained matrix of clay, bioclasts, and calcareous nannofossils. The conic reef body have been kept growing due to sediment buffering by branching colonies of L. pertusa, which were fragmented by intensive bioerosion before the burial. The deep-water coral reefs share fine-grained sediment composition and conic geometry with the Phanerozoic carbonate mud mounds, for which several different hypotheses have been proposed to explain their origins. They also resemble to the uppermost Jurassic-lower Cretaceous limestones in Japanese Islands in abrupt appearances within stratified non-carbonate sediments. Further researches are expected to provide significant insights for understanding environmental settings and processes of these ancient deposits, as well as the deep-water reefs themselves.