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Site U13171

Expedition 307 Scientists2

Background and objectives

Site U1317 is located on the northwest shoulder of Challenger Mound (51°22.8′N, 11°43.1′W; 781–815 m water depth). Challenger Mound is a part of the Belgica mound province, where outcropping and buried mounds occur in water depths of 600–900 m on the eastern slope of Porcupine Seabight on the southwest continental margin of Ireland (Fig. F1). The mound has an elongated shape oriented along a north-northeast to south-southwest axis and is partially buried under Pleistocene drift sediments. More sediment trapping occurs on the eastern upslope side, leaving only 30 m of Challenger Mound exposed. On the downslope side (location of Site U1316), less sediment trapping and erosive northward-flowing currents leave 70–80 m of the mound exposed (Van Rooij et al., 2003). An oval plateau demarcates the summit; however, the slopes of the mound grade steeply at an angle of 20°–33°. Present-day water masses in Porcupine Seabight flow generally northward and include European North Atlantic Water (ENAW) down to ~750 m. The ENAW overlies a core of Mediterranean Outflow Water (MOW) that reaches to ~1500 m and is marked by a salinity maximum and oxygen minimum. North East Atlantic Deep Water lies below 1500 m (Rice et al., 1991; White, 2001). De Mol et al. (2002) hypothesize that MOW plays an important role in the distribution of azooxanthellate cold-water corals found living on the flanks of Porcupine Seabight.

The mounds of Porcupine Seabight, including the mounds of the Belgica mound province, are renowned for their coral cover. On some mounds, living deep-sea corals Lophelia pertusa and Madrepora oculata often form dense thickets (De Mol et al., 2002). Challenger Mound, however, has little to no live coral coverage. Surface sediments near the top of the mound consist principally of sediment-clogged dead coral framework, coral rubble, and other skeletal remains of various organisms (Foubert et al., 2005). Shallow piston coring (Marion Dufresne Core MD01-2451G) recovered on-mound sediments of the uppermost 12.8 meters below seafloor (mbsf) that mostly consist of coral floatstone. The supporting carbonate matrix principally consists of calcareous nannofossils interspersed with occasional layers of siliciclastic material ~1 m thick (Foubert et al., 2005).

Two of the previously described seismic units, P1 (mound substratum) and P3 (mound and enclosing sediments), are encountered at this site (see “Physical properties” in the “Site U1316” chapter). Challenger Mound roots on the regional erosional unconformity separating seismic Units P1 and P3 (Fig. F2; see Fig. F3 in the “Expedition 307 summary” chapter). The mound appears on seismic profiles as an almost acoustically transparent dome-shaped structure. The mound is bounded by diffraction hyperbolae originating at the summit of the mound. Inside the mound, no internal reflectors have been recognized, indicating a uniform facies without any large acoustic impedance differences. The mound acoustic facies might also be interpreted as a loss of seismic energy due to scattering or absorption by the rough seabed and internal structure of the mound. However, an important observation is that the reflectors underneath the mounds show reduced amplitudes, although the reflectors do not completely disappear. Thus, not all the seismic energy is absorbed or dispersed inside the mound facies (De Mol et al., 2005). Analysis of velocity pull-ups of single-channel seismic indicates that the seismic facies of the coral banks is homogeneous and transparent with an estimated internal velocity of 1850 ± 50 m/s (De Mol et al., 2002). This velocity suggests a carbonate-rich sediment (velocity = 2300 m/s) intermixed with terrigeneous material (velocity = 1700 m/s), as groundtruthed by the surficial sediment samples (Foubert et al., 2005).

The substratum of the mound is characterized by a set of clinoforms formed by a number of superposed sigmoid reflectors of seismic Unit P1 (Fig. F2). These clinoforms are frequently characterized by a high-amplitude top sigmoid reflector. This seismic facies is interpreted as migrating drift bodies (Van Rooij et al., 2003; De Mol et al., 2005). The high amplitude and the reversals of signal polarity of these top sigmoid-reflectors indicate that these clinoforms could have contained traces of gas (Henriet et al., 2002) or a remarkable change in lithology (De Mol et al., 2005).

A two-dimensional basin model predicts a potential focusing of gas migration toward the Belgica mounds where the Cretaceous–Tertiary permeable layers pinch out beneath the mound area (see Fig. F4 in the “Expedition 307 summary” chapter) (Naeth et al., 2005). Modeling results based on seismic lines of industrial origin and six exploration wells indicate that Jurassic and older source rocks are mature to overmature throughout the basin. The reconstruction shows that seeping gas may have been available for methanotrophic bacteria and related formation of hardgrounds since the Miocene.

Based on the seismic profiling, Henriet et al. (2002) suggested a developmental model of Challenger Mound that consists of four stages. The initial stage may relate to fluid venting and authigenic carbonate precipitation, which provided a hardground for corals to colonize. The second stage involves the catalyzation of the mound growth by the settling of corals on the hardground in connection with microbiologically induced carbonate precipitation. Then, further growth of corals developed a carbonate mound in the third stage. Pelagic ooze and current-transported siliciclastic sediments were trapped in the framework of branching corals. In the final stage, Challenger Mound was buried asymmetrically with drift sediments.

An alternative hypothesis for the initiation and mound development invokes oceanographic control. Strong bottom currents in the Belgica mound province that result from internal tidal wave effects at the boundary between MOW and ENAW provides suspended food for filter-feeding cold-water corals, sweeps the polyps clean of detritus, and prevents burial of the corals by sediment. Initiation and development of coral growth can be related to major oceanographic changes in the Pliocene subsequent to the closure of the Isthmus of Panama (De Mol et al., 2002, 2005; Von Rooij et al., 2003).

Scientific drilling of Challenger Mound was the central objective of Expedition 307. Specific objectives of drilling Site U1317 were as follows:

  1. Establish what kind of surface the mound was built on; for instance, whether the mound base is on a carbonate hardground of microbial origin and whether past geofluid migration events acted as a prime trigger for mound genesis.
  2. Describe stratigraphy, lithology, and diagenetic characteristics for establishing the principal depositional model of cold-water coral banks including timing of key mound-building phases. This will help answer the question of whether Challenger Mound provides a present-day analog for understanding the Phanerozoic reef mounds.
  3. Define the relationship, if any, between the mound-developing event and global oceanographic events that might have formed erosional surfaces displayed on high-resolution seismic profiles. Cores penetrating these unconformities have to be analyzed by means of high-resolution stratigraphy. The well-established biostratigraphy for the Neogene marine sections of the North Atlantic will support interpretations of unconformities. A series of well-established proxies will be used to reconstruct high-resolution paleoclimate changes. In addition, the mound can be a high-resolution paleoclimate recorder because of its high depositional rate and contents of organic skeletons.
  4. Analyze geochemical and microbiological profiles that define the sequence of microbial communities and geomicrobial reaction throughout the drilled sections. This will also prepare the basic information to understand the diagenetic processes within the mounds. As putative biological constructions associated with geofluids, mounds may provide unique insight into the deep biosphere.

1 Expedition 307 Scientists, 2006. Site U1317. In Ferdelman, T.G., Kano, A., Williams, T., Henriet, J.-P., and the Expedition 307 Scientists. Proc. IODP, 307: Washington, DC (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.307.104.2006

2 Expedition 307 Scientists’ addresses.

Publication: 14 October 2006
MS 307-104