Porcupine seabight
(kindly contributed by Daniel Praeg & Veerle Huvenne)
The Porcupine Seabight is one of a number of deep-water basins on the continental margin west of Ireland, formed by crustal extension that culminated in the opening of the Atlantic Ocean ca. 100 million years ago. Subsidence since that time has resulted in water depths of up to 3000 m and, beneath the margins, sediment sequences up to 10 km thick that include significant hydrocarbon reservoirs (e.g. Shannon and Naylor, 1998). The long-term significance of sediment transfer from the adjacent continental landmass is attested to by the seabed channel and canyon systems that transect the northern and eastern slopes of the Seabight. However, the regional importance of alongslope transport by currents is equally apparent in hundreds of metres of drift sediment observed along the European margin, inferred to record deposition from northward flowing currents (Rice et al., 1991; New et al., 2000), notably the Eastern North Atlantic Water (above 750m depth) and the Mediterranean Outflow Water (750-1000m depth).
The specific conditions of both downslope sediment supply and alongslope current transport are known to have changed in response to glacial-interglacial cycles of climate change over the last several million years. This corresponds to the timescale of interest for growth of the carbonate mounds that are the focus of the survey.
Literature
- New, A.L., Barnard, A., Hermann, P. and Molines, J.-M., 2000. On the origin and pathway of the saline inflow to the Nordic Seas. Progress in Oceanography, submitted.
- Rice, A.L., Billet, D.S.M., Thurston, M.H. and Lampitt, R.S.,1991. The Institue of Oceanographic Sciences biology programme in the Porcupine Seabight: background and general introduction. Journal of the Marine Biology Association of the U.K., 71: 281-310.
- Shannon, P.M. and Naylor, D., 1998. An assessment of Irish offshore basins and petroleum plays. Journal of Petroleum Geology 21(2):125-152.
Carbonate Mounds
(kindly contributed by Daniel Praeg & Veerle Huvenne)
Carbonate mud-mounds, or deep-sea coral banks, have been found at several locations in the Porcupine Seabight, both at seabed and buried in the subsurface. Seabed investigations suggest them to consist of fine-grained sediment trapped by a framework of deep-sea cold-water corals (mainly Lophelia pertusa and Madrepora oculata). In general the mounds are found in depths of 500-1000 m and rise up to 200 m above the seabed, over areas of a few square kilometers. Seismic profiles show their growth to have been accompanied by the deposition of comparable thicknesses of contourite drift.
The reasons for their formation and the dynamics of their growth remain to be understood. One set of hypotheses links their formation to the seepage of hydrocarbons, either along faults (from deeper reservoirs) or from former gas-hydrate layers, possibly as a response to glacial-interglacial changes in current patterns and sea levels (Hovland et al., 1994; Henriet et al., 1998). Another hypothesis relates their distribution to nutrient fluxes driven by specific oceanic conditions, notably the interaction of internal waves, formed at the boundary between different water masses, with the continental slope (De Mol et al., 2000).
The mounds occur in provinces, each with distinctive characteristics; those of the study area have been designated the Belgica province. To date, their distribution and character have been investigated using widely-spaced seismic profile data, locally complemented by sidescan imagery, shallow cores and video. The multi-beam swath bathymetry survey is intended to obtain the first 3-dimensional picture of the seabed form and distribution of the Belgica mounds and, in combination with seismic profile data, to clarify the relationship of the mounds to the surrounding drift sediment. The results will provide important constraints on models of mound formation and growth in relation to alongslope and downslope sediment transport processes on the continental slope.
Literature
- De Mol, B., Van Rensbergen, P., Pillem, S., Van Rooij, D., McDonnel, A., Ivanov, M., Swennen, R. and Henriet, J.-P, 2000. Larger deep-water coral banks in the Porcupine Basin, southwest of Ireland. In preparation
- Henriet, J.-P., De Mol, B., Pillen, S., Vaneste, M., Van Rooij, D., Versteeg, W., Croker, P.F., Shannon, P.M., Unnithan, V., Bouriak, S., Chachkine, P. and the Porcupine Belgica 97 shipboard party, 1998. Gas hydrate crystals may help build reefs. Nature, 391: 648-649.
- Henriet, J.-P., De Mol, B. and the Porcupine-Belgica '97 & '98 Shipboard Parties, 1999. Carbonate mounds, ring bioherms and past slope failures in the Porcupine Basin: prologue to a far- reaching story? In P.F. Croker and O. O'Loughlin (eds.), The Petroleum Exploration of Ireland's Offshore Basins; Extended Abstracts: 90-92.
- Hovland, M., Croker, P.F.and Martin, M. 1994. Fault-associated seabed mounds (carbonate knolls?) off western Ireland and north-west Australia. Marine and Petroleum Geology, 11(2):232-246.
