Fr1134/14-1

The DFG project “Cold-water coral mound genesis: IODP Leg 307 and Mediterranean occurrences” concentrated on the fascinating cold-water coral (CWC) carbonate factory, which can be found along the worlds continental margins, especially in the N Atlantic. The project focused on the interplay of biological (coral growth), sedimentological (accumulation/erosion) and palaeoclimatic (glacial/interglacial) processes, which have generated giant CWC mounds at aphotic depths and cold waters. Therefore, two examples were studied in detail: (i) Challenger Mound, a giant cold-water coral mound in the Belgica Mound Province, Porcupine Seabight, SW Irish shelf, and (ii) the potential only fossil cold-water coral mound exposed on land at La Montagna, Messina, Sicily. Challenger Mound was drilled during IODP Leg 307 and altogether, four on-mound cores were recovered. Major changes in Challenger Mound development (e.g. start, crisis, accumulation rates) coincide with global climatic and oceanographic turnarounds, such as the northern hemisphere glaciation and the mid-Pleistocene transition and suggest an external (climatic) control of mound development. The major growth of Challenger Mound (deposition of the 132-m-thick basal unit M1) took place from about 2.7 Ma to about 1.6 Ma and occurred while non-deposition or even erosion prevailed in surrounding areas. This highlights the potential of the CWC ecosystem to create its own accommodation space and to baffle background sediment effectively. Consequently, CWC mounds should be seen as potential palaeoceanographic archives on continental slopes where erosion prevailed in the time interval of interest. A fact, which may also account for other mound systems. The carbonate accumulation rate of Challenger Mound is estimated with 17.3 g/(cm2×ka) in unit M1 and 5.7 g/(cm2×ka) in unit M2, which lasted from 0.8 to 0 Ma. The reduced accumulation rates in mound unit M2 is due to the abundant occurrence of unconformities, an indication for a predominant discontinuous mound growth during this phase. The carbonate accumulation rates are about 4 to 12 % of the carbonate accumulation rates of tropical shallow-water reefs but exceed the carbonate accumulation rates of slopes by a factor of 3.9 to 11.8. Consequently, CWC mounds can be classified as carbonate, hence CO2, sink. The diagenesis of Challenger Mound is generally destructive. The dissolution of the aragonite and the lack of early marine lithification might result in a mud-dominated carbonate mound that exhibit no evidence for bafflers, binders, or framework builders. This observation highlights how difficult the environmental interpretation of fossil carbonate mounds can be. First results from La Montagna outcrop exhibit two major mound development stages (sequences), which are separated by unconformities. Several minor unconformities within these two mound stages were identified. The corals are predominantly dissolved, but an early matrix lithification secured their preservation as moulds. Only in patches well-preserved corals occur. Rarely the dominating coral rudstone facies is intersected by thin layers, which are dominated by fragments of lepadid and balanid cirripeds, or by large terebratulid brachiopods. Sequence/unconformity formation is believed to be controlled by global sealevel fluctuations, which had a major effect on the current system within the Strait of Messina. Conclusively can be said that the two study sites occur in generally different environmental settings: (i) Challenger Mound developed on a passive continental margin while the La Montagna mounds developed in a very active tectonic setting on the flanks of a graben structure; (ii) Challenger Mound developed under the influence of a water mass boundary between the Eastern North Atlantic Water (ENAW) and the Mediterranean Outflow Water (MOW) while the La Montagna mounds were influenced by a complex current system within the Strait of Messina, which connects the Tyrrhenian and Ionian Sea. But both mound systems exhibit major changes in mound development (start, crisis, formation of unconformities) most likely linked to global climate (sea level) fluctuations, which is interpreted to be the consequence of highly effected current systems controlling CWC mound growth.