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doi:10.2204/iodp.proc.307.101.2006

Introduction

The well known warm-water coral reefs developed in shallow-water tropical to subtropical regions with typical carbonate depositional environments. However, the discovery of large-scale deepwater mounds associated with cold-water corals extending along the northeastern Atlantic continental margins raises interesting questions about their structure, origin, and development. Cold-water coral banks from depths where light cannot reach were first described by Teichert (1958) off European coastlines. He drew the attention of geologists to the fact that coral frameworks thrive in high latitudes and/or deep environments and emphasized the similarities with tropical coral reefs. Neuman et al. (1977) discovered that benthic invertebrates including corals and crinoids construct mounds (called lithoherms) several hundred meters wide and 50 m high at 600–700 meters below sea level (mbsl) along the Straits of Florida. They were considered to be an exceptional case of coral development in deep environments, but contemporaneously similar structures had been recognized in numerous seismic profiles collected from the northeast Atlantic.

Currently, the mounds associated with cold-water azooxanthellate corals are known to occur at depths to 1500 m along continental margins and shelves of the northeast Atlantic margin from Mauritania to Arctic Norway (Freiwald et al., 2004). High-resolution seismic surveys revealed that the mound structures can reach widths of several kilometers (Henriet et al., 1998; De Mol et al., 2002) and heights of 350 m (Kenyon et al., 2003). Among the northeast Atlantic, the most intensively studied cold-water coral buildups are located in the Norwegian shelf, Rockall Trough and Porcupine Seabight, southwest of Ireland. Along the Norwegian shelf, extending from 62°–71°N, cold-water corals form reefs up to 30 m high at 200–400 mbsl. The highest densities and largest continuous reefs occur along the continental shelf break and on the edges of shelf-crossing trenches and morainic formations (Freiwald et al., 2004).

The Porcupine Seabight area surpasses the Norwegian offshore area in terms of size and frequency of cold-water coral buildups. Sixty-six Belgica mounds have been mapped by means of swath bathymetry and two-dimensional (2-D) seismic. In the Hovland mound province, 39 mounds have been identified. In total, 641 Magellan mounds over a total area of ~1500 km² have been mapped by 2-D and three-dimensional (3-D) seismics (Huvenne et al., 2003). This estimation of the mound population is underestimated because the 2-D seismic grid is not dense enough to cover all the mounds in the area. Based on the mound density in this province (1 mound/km²), it is estimated that there are at least 1500 Magellan mounds. In total, 1600 mounds could appear in Porcupine Basin. Recently, more mounds have been discovered in the northern mound provinces of Porcupine Seabight, which will significantly increase the total number of mounds in the basin (De Cock, 2005).

The mounds reach heights of up to 250 m and widths of up to 5 km and are enclosed by siliciclastic contourites (Van Rooij et al., 2003; Huvenne et al., 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 they are commonly colonized by a diverse biota including deep-sea corals Lophelia pertusa and Madrepora oculata and other invertebrates (Foubert et al., 2005; Huvenne et al., 2005). Despite this diverse and large data set, knowledge of the internal structure, initiation, and growth of these impressive seafloor features within Porcupine Seabight has remained elusive. Explanations of the origin and evolution of the Porcupine mounds revolve around two scenarios that can be considered as either competing or complementary hypotheses: (1) oceanographic and environmental conditions control mound initiation and growth (e.g., De Mol et al., 2002, 2005a; Huvenne et al., 2003, 2005), 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 condition stimulating mound development is the interaction of hydro- and sediment dynamics. The position and strength of currents provide not only the nutrients for coral and other benthic biota but also clear stable substrates for settlement of coral larvae and inhibit the smothering of coral by too much sediment (Frederiksen et al., 1992 [Faroe Islands]; 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; White, 2001). 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 today at ~800 meters below seafloor (mbsf) (White, 2001). A strong current is thought to provide suspended nutrients to filter-feeding cold-water corals, sweep the polyps clean of detritus, and protect 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 rerouted the huge warm-water mass of ENAW northward 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 cold-water coral mounds thus might have been related to the establishment of MOW and/or ENAW, introducing cold-water coral larvae to the northeast Atlantic (De Mol et al., 2002, 2005a).

The hydrocarbon seepage hypothesis in Porcupine Seabight was first proposed by Hovland et al. (1994). Microbially mediated oxidation of hydrocarbons may provide favorable conditions for cold-water corals by increasing levels of inorganic carbon for skeletal accretion (Hovland et al., 1998) as well as providing a stable substrate for submarine lithification. The linear distributions seen in some mounds of Porcupine Seabight suggests that the mounds were established along linear structures, such as faults (Hovland et al., 1994). A study by Bailey et al. (2003), however, based on two- and three-dimensional seismic data, found no correlation between mound locations and fault lines. Henriet et al. (2001) suggested that conditions in the Magellan mound province during glacial periods were probably suitable for gas hydrate formation and that the decomposition of gas hydrates 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 could 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 subsequent coral growth.

The Atlantic cold-water coral mounds have many features in common with numerous Phanerozoic mud mounds. The true dawn of carbonate mud mounds lies in the Cambrian, when mounds suddenly featured diverse 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 the mid- to late Ordovician, 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 prominent 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 an isotopically light hydrocarbon source. 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 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 of metazoan framework reefs (Riding, 1991). Most Cenozoic mud mounds appear to be of biodetrital origin. Scientific drilling during Ocean Drilling Program (ODP) Leg 182 confirmed the existence of bryozoan reef mounds buried in the cold-water carbonate platform sediments at 200–350 mbsl in Great Australian Bight (Feary et al., 1999; James et al., 2000). These mound complexes consist of unlithified floatstone structures, rich in zooidal bryozoans that were still growing during the last glacial lowstand. It remains to be determined if the Porcupine mounds are present-day analogs of the Phanerozoic reef mounds and mud mounds, for which depositional processes and environmental settings are not fully understood.

Integrated Ocean Drilling Program (IODP) Expedition 307 was proposed to recover sediment in order to groundtruth the origin and evolution of the cold-water coral mounds in Porcupine Seabight. A wide range of lithostratigraphic, biostratigraphic, magnetostratigraphic, and physical property data will be integrated to evaluate sedimentary processes and paleoceanographic history. The role of microbes in mound initiation and development will be examined by incorporating geochemistry and microbiology. Microbiological profiles will define the sequence of microbial communities and geomicrobial reaction throughout the drilled sections and provide basic information to understand diagenetic processes within the mounds.

Cold-water coral mounds may also be potential high-resolution environmental recorders. Mound section lithology and bioclastic composition might record paleoenvironmental climate changes because cold-water corals are sensitive to conditions such as water temperature, salinity, and current strength. Furthermore, coral skeletons themselves are useful paleoproxy recorders: ¹⁴C and U/Th ratios have been used to reconstruct changes in deep-sea circulation (Frank et al., 2004) and Cd/Ca ratios to reconstruct nutrient contents (Adkins et al., 1998). Cold-water corals grow as fast as 25 mm/y (Rogers, 1999), fast enough to apply the same methods of coral climatology used on tropical–subtropical reef-forming corals. Stable isotope and trace element data are expected to provide environmental constraints.

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