From sediment to rock: diagenetic processes of hardground formation in deep-water carbonate mounds of the NE Atlantic

Abstract

Modern cool-water carbonate mounds topped by corals form an extended reef belt along the NW European continental margin at 200-1200 m water depth. An essential element of mound growth are hardgrounds which provide a stable substratum for mound-building invertebrate colonisation and stabilise the inclined mound flanks. Evaluating the degree of lithification and the slope stability against erosion represents an important task within the ESF programme MOUNDFORCE under the umbrella of EUROMARGINS. Sampling of hardgrounds during RV Meteor cruises M61-1 and -3 in 2004 by means of the IFM-GEOMAR TV-grab and the Bremen ROV QUEST focused on carbonate mounds of the Porcupine Seabight and northwestern Rockall Bank off Ireland. Lithified carbonates of mid-Pleistocene age were exhumed during the Holocene and are now exposed on the top and flanks of numerous carbonate mounds showing a patchy to dense colonisation by living corals and associated invertebrates. The sediments, composed of foraminiferal-nannoplankton oozes and admixed mound-derived invertebrate skeletons, range from partly lithified chalks to dense micritic limestones. These wackestones to packstones clearly differ from bacterially induced authigenic carbonate crusts typical of hydrocarbon seep settings by showing current-induced sedimentary structures, a non-luminescing matrix indicating oxic pore fluids, and a marine isotopic signature lacking any depleted carbon regime which is typical of anaerobic methane oxidation. The carbonate lithification is driven by carbonate ion diffusion from supersaturated seawater into the pore fluids S. Noé ( ) · W.-C. Dullo in the studied areas. Vigorous bottom currents were the ultimate control not only of carbonate cementation by enhancing the diffusion process and supporting a pumping mechanism, but also of hardground formation and mound shaping by exhuming lithified carbonates and preventing finegrained sediment accumulation at the downslope mound flanks.

The Porcupine Bank Canyon coral mounds: oceanographic and topographic steering of deep-water carbonate mound development and associated phosphatic deposition

“…Moreover, the δ 13 C values show no significant depletion which could be related to seepage of hydrocarbon-rich fluids. This is consistent with the findings of Noé et al (2006) who obtained similar δ 18 O and δ 13 C values for carbonate hardgrounds from the Propeller Mound in the Porcupine Seabight. The association of coral reefs, carbonate deposits and methane seepages therefore remains a speculative issue at this stage.…”

“…Phosphate 'nodules' and intraclasts were drilled close to the base of the Magellan carbonate mound (Expedition-Scientists-ODP-307 2005). Furthermore, glauconite has been observed to occur in the upper bathyal coral zone of the Rockall Bank (Scoffin et al 1980), in the hardgrounds near Connacht Mound (Noé et al 2006), in bathyal gorgonian skeletons from the Bay of Biscay margin (Noé et al 2007), on the Sula Reef of the mid-Norwegian shelf , and in Gonicorella thickets of the Chatham Rise, New Zealand (Dawson 1984;Kudrass and von Rad 1984).…”

“…8d-f). Noé et al (2006) and Mienis et al (2009) possibly describe similar authigenic deposits from carbonate mounds of the NE Atlantic, which contain carbonate-cemented layers with dissolved coral branches.…”

Franken Mound: facies and biocoenoses on a newly-discovered “carbonate mound” on the western Rockall Bank, NE Atlantic

“…The current strength here probably exceeds those of other parts on the Franken Mound and has an erosive, and thus, shaping eVect on the mound indicated by a rough hardground surface and the presence of several cracks and break-ups. A petrological study of Noé et al (2006) revealed that the hardgrounds at the western Xank are composed of carbonates (pelagic micrites) which show an increasing lithiWcation from the upper Xank down to the base of the hardground escarpment. The carbonate lithiWcation in the NE Atlantic Ocean is predominantly inXuenced by the chemistry of the seawater (high alkalinity, supersaturation of low-magnesium calcite, poverty in phosphate and dissolved organic matter, high oxygen content), but strong bottom currents also play a controlling role for the lithiWcation of carbonates as they enhance the carbonate ion diVusion from seawater through the sediments .…”

Cold-water coral growth in relation to the hydrography of the Celtic and Nordic European continental margin

“…For example, seamounts and canyons are areas with pronounced vertical relief with hard substrata providing ideal settling sites for corals, often associated with a strong current regime and therefore low sediment deposition (Herring 2002). The scleractinian Lophelia pertusa has been recorded on morainic ridges, lithified sediments, and vertical cliffs (Freiwald et al 1999, Noé et al 2006, but also in coarse sand habitats (Foubert et al 2005, Huvenne et al 2005. It is assumed that the coral initially settles on a hard substrate such as a pebble or a shell (e.g.…”