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doi:10.2204/iodp.pr.307.2005
Carbonate mounds and reefs are a fundamental and recurrent expression of life in the geological record from Precambrian times onward. The true dawn of carbonate mud mounds is in Cambrian times, when mounds suddenly featured a diversity in microbial and biodetrital fabrics with abundant mound-building calcified microbes (Riding, 1991), calcareous algae, and a variety of Paleozoic benthic invertebrates that may have played an ancillary role in mound construction. In mid- to late Ordovician times, the dramatic rise of large skeletal metazoans such as stromatoporoids, corals (Rugosa and Tabulata), and bryozoans, as well as higher algae, paved the way for the strong development of reefs and typical stromatactoid mud mounds. Lower Devonian (Gedinnian) mounds in the Montagne Noire, France, exhibit the most spectacular stromatactis fabrics, interpreted as the result of decaying microbial mats (Flajs and Hüssner, 1993). Stratigraphically younger (Emsian) conical carbonate mounds (kess-kess) of the Moroccan Anti-Atlas are related to precipitation from hydrothermal fluids (Kaufmann, 1997), some of which are inferred to be related to a light carbon source (hydrocarbon). Some of the most impressive of early Carboniferous bank aggregates, as thick as 1 km, are those known as the Waulsortian reefs (Lees, 1988; Somerville, 2003). In full Mesozoic times, declines in the abundance and diversity of microbial mounds are recorded from the Triassic to the Cretaceous. From the mid-Cretaceous onward, microbial fabrics are only known as components to metazoan framework reefs (Riding, 1991). Most Cenozoic mud mounds are of biodetrital origin, although microbial components might have remained significant in deeper water. Scientific drilling (Ocean Drilling Program [ODP] Leg 182) confirmed the existence of bryozoan reef mounds buried in the cold-water carbonate platform sediments at 200–350 meters below sea level (mbsl) in Great Australian Bight (Feary et al., 1999; James et al., 2000). These mound complexes consist of unlithified floatstone structures, rich in zooidal bryozoan forms that were still growing during the last glacial lowstand.
Coral reefs are commonly considered to have developed in shallow-water tropical to subtropical regions with typical carbonate depositional environments. However, the discovery of large-scale deepwater carbonate mounds extending along the northeastern Atlantic continental margins as well as the Australian bryozoan reef mounds in cool-water slope environments has cast new interest on their structure, origin, and development. The Atlantic coral mounds from depths where light cannot reach were first described by Neuman et al. (1977). They discovered that benthic invertebrates including corals and crinoids construct mounds several hundred meters wide and 50 m high at 600–700 mbsl along the Straits of Florida west of the Bahamas. They were considered to be an exceptional case of coral reefs developed in amazingly deep environments, but contemporaneously similar structures had been recognized in numerous oil industry seismic profiles collected from the northeast Atlantic. Scientific research on deepwater coral mounds was stimulated by the release of some of these profiles in the mid-1990s. Oceanographic exploration using side-scan sonar, underwater video imagery, and drag sampling has collected information on bottom sediments and indicated that the structures are really formed by benthic communities with cold-water corals. Currently, the deepwater coral banks and mounds are known to occur at depths to 1500 m on shelves, shelf margins, and seamounts of the northeast Atlantic from Morocco to Arctic Norway. Additional high-resolution seismic surveys revealed that the mound structures can reach a width of several kilometers (Freiwald et al., 2004) and a height of 350 m (Kenyon et al., 2003). Exploration for deepwater coral banks was recently expanded outside of Atlantic areas and showed their possible global distribution.
Among the deepwater mound provinces, the most intensively studied provinces are offshore of Norway, Rockall Trough, and Porcupine Seabight located southwest of Ireland. Along the Norwegian continental slope extending from 62°–70°N, cold-water corals form banks or mounds up to 40 m high at 400–500 mbsl. They appear to develop on stable substrate formed of the exposed Neogene sedimentary rock and boulders derived from fjords (Freiwald et al., 1997).
The Porcupine Seabight area surpasses the Norwegian offshore area in terms of size and frequency of the mounds and is the most important Atlantic mound area. Seismic studies have revealed more than 1500 carbonate mounds in the Magellan mound province, and it is estimated that there are ~2000 mounds in Porcupine Basin (De Mol et al., 2002; Huvenne et al., 2003) that generally exhibit conical geometries surrounded or covered by siliciclastic contourites. They can reach as high as 250 m and as wide as 5 km (Huvenne et al., 2002, 2003; De Mol et al., 2002). The mounds in Porcupine Seabight have been the focus of more than 20 cruises in the last decade. Sediments and video images collected on the seafloor indicate that the mounds are commonly colonized by various biota including deep-sea corals Lophelia and Madrepora and other invertebrates (Foubert el al., 2005). However, details on the internal structure, initiation, and growth of these impressive seafloor features within Porcupine Seabight remained veiled. Explanations of the origin and evolution of the Porcupine mounds revolve around two scenarios that may be expressed as either competing or complementary hypotheses: (1) oceanographic and paleoenvironmental conditions control mound initiation and growth, and (2) hydrocarbon seepage initiates microbial-induced carbonate formation and indirectly fuels coral growth (endogenous control) (Hovland et al., 1998; Henriet et al., 2001). The oceanographic or environmental hypothesis states that the most important conditions stimulating mound development are the interaction of water currents and sediment dynamics. Enhanced currents provide nutrient for the corals and may sweep free stable substrates for settlement of coral larvae (Frederiksen et al., 1992; De Mol et al., 2002 [Porcupine]; Kenyon et al., 2003 [Rockall]; Freiwald et al. 1997 [Norway]; Colman et al., 2005 [Mauritania]). Enhanced bottom currents at tidal frequency (Pingree and Le Cann, 1989, 1990) are observed in the Porcupine mound provinces (Rice et al., 1991). They may result from the internal tides at the boundary between water masses of different densities, such as between Mediterranean Outflow Water (MOW) and Eastern North Atlantic Water (ENAW), located at ~800 meters below seafloor (mbsf) at present (White, 2001). A strong current provides suspended food to filter-feeding cold-water corals, sweeps the polyps clean of detritus, and protects the corals from sediment burial. Initiation of mound development has been linked to global paleoceanographic change. Closure of the Isthmus of Panama at ~4.6 Ma deviated the huge warm-water mass of ENAW and increased deepwater advection and stratification in the Atlantic (Haug and Tiedemann, 1998). In combination, MOW resumed after the late Miocene–Early Pliocene salinity crisis in the Mediterranean (Maldonado and Nelson, 1999). The oldest fossil records of Lophelia and Madrepora were reported from the Mediterranean area in the Early Pliocene. Initiation of the deep-sea coral mounds might have been related to establishment of MOW and/or ENAW, which introduced larvae of the cold-water corals to the northeast Atlantic in a manner similar to the coral banks established on the Nordic continental shelf break (Henriet et al., 1998).
The seepage hypothesis was first proposed by Hovland et al. (1994), who correlated the distribution between coral mounds with areas showing high dissolved light hydrocarbon contents in water. Hydrocarbon seepage may prepare favorable conditions for deep-sea corals, in terms of raised inorganic carbon for skeletal accretion (Hovland et al., 1998) and for submarine lithification providing stable substrate. The aligned occurrence seen in some mounds of Porcupine Seabight suggests that the mounds were established along linear structures, such as faults (Hovland et al., 1994). Bailey et al. (2003), however, studied the Magellan mound province in detail from two-dimensional and three-dimensional (3-D) seismic data and found no correlation between mounds and fault locations. Henriet et al. (1998) suggested that conditions in the Magellan province during glacial periods were probably suitable for gas hydrate formation and that decomposition of the gas hydrate could trigger a submarine slide. Later phases of gas seepage would then be focused by this buried slide to specific locations, causing the ringlike structure in the Magellan mounds. The oceanographic and fluid seepage scenarios may have acted in a complementary fashion: microbially induced carbonate hardground formation associated with gas or hydrocarbon seepage provided the hard substrate required by the settling coral larvae. Once established, further development of the mound was subject to oceanographic conditions suitable for further coral growth. The presence of corals covering the mound may be only fortuitous. The mounds themselves may be composed of microbial automicrite, like well-known Phanerozoic carbonate mounds, but only recently covered by cold-water corals. Coral growth might even have led to accelerated burial of the mound structures through enhanced baffling and trapping of sediment. Conclusive evidence for any of these theories is still missing. Only scientific drilling through the core of a mound structure will provide clearer insight to the formation and development of these enigmatic structures. Integrated Ocean Drilling Program (IODP) Expedition 307 was proposed to obtain evidence for understanding the origin and evolution of the deepwater carbonate mounds in Porcupine Seabight. Sedimentary processes and paleoclimatic history are recorded in lithostratigraphic, biostratigraphic, magnetostratigraphic, and physical property data. In addition, the mounds may be biological constructions associated with hydrocarbon-rich fluids and therefore provide a unique habitat for the deep biosphere. Geochemical and microbiological profiles would define the sequence of microbial communities and geomicrobial reaction throughout the drilled sections and provide basic information to understand diagenetic processes within the mounds.
The Atlantic deep-sea carbonate mounds share geometric features and occurrences with numerous Phanerozoic mud mounds of different ages formed under the control of microbial processes. Perhaps the Porcupine mounds are present-day analogs for the Phanerozoic reef mounds and mud mounds, for which depositional processes and environmental settings are not fully understood. Or are these mounds North Atlantic equivalents to the Quarternary bryozoan mounds found in Great Australian Bight? Limestone bodies that contain coral skeletons are usually interpreted as indicating warm and shallow water; however, the abundance of deepwater coral mounds mean such paleoenvironmental interpretation may not be so simple.
Deepwater coral mounds may also be potential high-resolution environmental recorders. Mound section lithology and bioclastic composition might record glacial–interglacial climate changes because cold-water corals are sensitive to conditions such as water temperature and current strength. Furthermore, coral skeletons themselves can be used for analysis: 14C and Cd content were used to reconstruct changes in deep-sea circulation (Frank et al., 2004) and nutrient contents (Adkins et al., 1998), respectively. Cold-water corals grow as fast as 25 mm/y (Rogers, 1999), fast enough to apply the same methods of the coral climatology using tropical–subtropical reef-forming corals. Analyses of stable isotopes and trace elements are expected to provide temperature and carbon circulation data at a subannual resolution.