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doi:10.2204/iodp.pr.307.2005
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. F2). Two-dimensional basin modeling has been used to evaluate the possible link between hydrocarbon leakage and mound growth (Naeth et al., 2005). Seismic lines of industrial origin and six exploration wells were used to calibrate the burial and thermal history using vitrinite reflectance, bottom hole temperatures, and apatite fission track data. 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 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. Only Aptian and Tertiary deltaic layers direct hydrocarbon flow out of the basin. The model predicts a potential focusing of gas migration toward the Belgica mounds area, where a pinchout of Cretaceous and Tertiary layers beneath the mound area is observed. 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.
Sedimentary buildup might have been controlled by microbial communities that may have played an active role in stabilization of the steep flanks and in possible lithification of the mound core through automicrite formation. On the other hand, 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 distinct settings (De Mol et al., 2002; Van Rooij, 2004; Foubert et al., 2005; Wheeler et al., 2005; Huvenne et al., 2005). These mounds cluster in the highest salinity water 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, enhanced currents associated with mixing and interaction of water masses featuring a density contrast may provide effects for coral growth such as enhanced fluxes of potential nutrients and low sedimentation rates. Such observations consequently also argue for a complex but equally important external control. A central hypothesis to be tested is to what extent mound provinces originate at the crossroads of fluxes of internal (trigger phase) and external (relay phase) origin (Henriet et al., 2002).
The thick drift sediment sheet embedding the mounds holds a high-resolution record of past fluctuations of water masses and currents on this section of the North Atlantic margin (Dorschel et al., 2005). Seismic records of exceptionally high resolution may allow correlation of this record to mound growth phases. Correlation of the Porcupine drift record with ODP sites along the Atlantic margin opens perspectives of cross-basin comparisons. Corals in the drill cores provide information on the paleoceanographic conditions, as already substantiated by pre-IODP coring results (Marion-Dufresne preparatory coring) (Foubert et al., 2004). Variations in terrigeneous content and organic matter in drift sediments should allow us to trace terrestrial sources and shelf-to-slope sediment pathways. The association of mounds and drifts on this upper continental slope thus also provides a unique paleoenvironmental record of the Atlantic margin.
The objectives of Expedition 307 were framed by five major hypotheses:
- 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 to what extent fluids may or may not have played a role in mound genesis and/or growth.
- 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.
- 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 will be used to study paleoenvironmental change including response to Pleistocene glacial–interglacial cycles.
- 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. Challenger Mound resembles the Phanerozoic mound in many aspects, including its size and geometry. Only 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 the mound development.