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doi:10.2204/iodp.proc.307.101.2006

Scientific objectives

External versus internal controls

The apparent coincidence between the presence of giant mound clusters and potentially deeper-lying hydrocarbon deposits suggests a possible internal control from mostly transient fluxes of geofluids in deep geological reservoirs to the seabed (Fig. F4). Two-dimensional basin modeling has been used to evaluate the possible link between hydrocarbon leakage and mound growth (Naeth et al., 2005). Industrial seismic lines and six exploration wells were used to calibrate the burial and thermal history using vitrinite reflectance, bottom hole temperatures, and apatite fission track data (Naeth et al., 2005). Modeling results indicate that Jurassic and older source rocks are mature to overmature throughout the basin. Cretaceous strata are immature to mature in the central part of the basin and immature on the flanks. The Tertiary sequence remains immature over the entire basin. Hydrocarbon generation started in Late Cretaceous times for the deepest sequences. Phase separation was modeled to occur during migration at depth ranges between 2000 and 4000 m. Upon phase separation, migration of a free gas phase dominated over that of oil, such that gas is the main migrating fluid in shallower intervals. Migration is mainly buoyancy-driven and vertical. The model predicts a potential focusing of gas migration upslope of the Belgica mounds area, where a pinchout of Cretaceous and Tertiary layers beneath the mound area is observed. Only Aptian and Tertiary deltaic layers direct hydrocarbon flow out of the basin. The reconstruction shows that seeping gas may have been available for methanotrophic bacteria and related formation of hardgrounds since the Miocene. Analysis of very high resolution seismic data below the Belgica mounds highlighted acoustic anomalies within the basal sigmoidal sequences (amplitude, instantaneous frequency, and polarity), possibly related to low quantities of gas.

Mound development may be controlled by microbial communities with automicrite formation playing a key role in stabilization of the steep flanks and lithification of the mound core. On the other hand, oceanographic processes may be more significant. These mounds are located on a margin that throughout the Neogene–Quaternary has repeatedly alternated between glacial and interglacial environments. There is also increasing evidence that active mound provinces also occur in oceanographically unique settings (e.g., De Mol et al., 2002; Van Rooij, 2003, 2004; Huvenne et al., 2003; Rüggeberg et al., 2005; Kenyon et al., 2003; Colman et al., 2005). For example, these mounds cluster in the highest salinity water masses and also bathymetrically coincide with the spread of the oxygen minimum zone along the deep continental margin (De Mol et al., 2002; Freiwald et al., 2004). In Porcupine Seabight, these specific environmental conditions are provided by the northward flow of MOW at intermediate depths (~700–900 m). Locally, turbulent mixing of water masses with density contrasts stimulate productivity and increase nutrient export, creating conditions favorable for coral growth (e.g., De Mol et al., 2005a). Such observations consequently argue for a complex but important external control. A central hypothesis to be tested is to what extent mound initiation and growth relies on internal versus external processes (Henriet et al., 2002).

Mounds and drifts

The thick drift sediment sheet enclosing the mounds holds a high-resolution record of past fluctuations of water masses and currents on this section of the North Atlantic margin (Van Rooij, 2004). Correlation of the Porcupine drift record with ODP sites along the Atlantic margin creates the potential for cross-basin comparisons. Corals in the drill cores provide information on paleoceanographic conditions, as already substantiated by pre-IODP coring results (Marion Dufresne preparatory coring) (Foubert et al., 2005). Variations in terrigeneous content and organic matter in drift sediments should allow us to trace terrestrial sources and shelf-to-slope sediment pathways.

Hypotheses tested

The objectives of Expedition 307 were framed by four major hypotheses:

1. Gas seeps act as a prime trigger for mound genesis: a case for geosphere–biosphere coupling. Drilling to the base of the mounds will allow verification of the extent to which fluids may or may not have played a role in mound genesis and/or growth.
2. Mound “events” (prominent erosional surfaces) reflect global oceanographic events. Erosional surfaces are displayed on high-resolution seismic lines. Holes penetrating these unconformities were analyzed by means of high-resolution stratigraphy. The well-established biostratigraphy for the Neogene marine sections of the North Atlantic support interpretations of the timing of the unconformities.
3. The mound may be a high-resolution paleoenvironmental recorder because of its high depositional rate and contents of organic skeletons. A series of well-established proxies can be used to study paleoenvironmental change including response to Pleistocene glacial–interglacial cycles.
4. The Porcupine mounds are present-day analogs for Phanerozoic reef mounds and mud mounds. There are still debates on depositional processes of ancient carbonate mounds that occur ubiquitously in Paleozoic–Mesozoic strata worldwide. The role of microbes in producing and stabilizing sediments has been especially acknowledged by a number of case studies in the last decade; however, conclusive evidence is still missing. Only scientific drilling provides significant information on stratigraphy, depositional age, sediment/​faunal compositions, and geochemical/​microbial profiles of the mound interior. These data sources together establish the principal depositional model of deepwater carbonate mounds and evaluate the importance of microbial activity in mound development.

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