Variability on Pleistocene to recent sedimentation from the Porcupine Seabight, northeastern Atlantic: Implications for Carbonate mound growth and development

Abstract

The Porcupine Seabight (PSB) on the Irish continental margin contains three distinct carbonate mound provinces with many of the individual mounds being colonised by the cold-water azooxanthellate corals Lophelia pertusa and Madrepora oculata. The oceanographic regime in the PSB is one of the main controls on the location and development of these mounds. For these sea floor structures bottom currents play an important role and nowadays in the PSB the Mediterranean Outflow Water (MOW) appears to be crucial for the living cold-water coral ecosystems on these carbonate mounds. The variability in present-day bottom current speeds on Propeller Mound were studied from modern sedimentation. The dominant current direction in the PSB is from south to north, although at Propeller Mound the currents aredeflected in a southwesterly direction. Grain size and compositional analyses of box core surfaces taken from the seafloor adjacent to and from Propeller Mound in theHovland Mound Province show that the intensity of these currents varies locally. Highest current speeds occur to the west of the mound and on the mound surface, winnowing away finer sediment fractions (clay-fine silt) and leaving coarse sortable silt and sand lag deposits. Coarse grains are absent from the seafloor to the east of the mound. Local forcing of bottom currents is likely due to the elevated topography of the mound and tidal currents in the area. Sediments from Propeller Mound contain elevated calcium carbonate contents (50 percentage) compared to background sediments (39 percentage)what is proportional to the volume of coarse bioclastic material present. The hydrodynamic variability recorded in Pleistocene drift sediments was studied in a more southerly mound province. Analysis of core MD-01-2450 from the BelgicaMound Province documents drift sedimentation in an off-mound location reaching far beyond 200 kyr B.P. An hiatus representing >160 kyr is present, separating an upper silty drift body containing abundant coarse ice rafted debris IRD) from a finer grained glacial unit below. This hiatus is represented as a coarse sandy unit interpreted as a debris flow or slump. The sedimentary record indicates that glacialperiods in the PSB were periods of reduced bottom water circulation, with variability in the size range of IRD delivered to the seafloor. Changes in the geological and biological record in Propeller Mound are related tochanges in hydrodynamics associated with glacial and interglacial cycles. Down-core variability in grain-size and bioclastic composition through Propeller Mound suggeststhat current speeds have not been stable in the on-mound environment through time. Sediments aged between 0.27 and 1.5 Ma (Marine Isotope Stages 9 to 50) show that the sediment composition on the mound has changed considerably and repetitively through time. Four sedimentary facies have been identified in the mound and are related to current intensities and glacial?interglacial cycles. Fine grained mudstones and wackestones are deposited during glacial periods and coarse coral grain- and packstones are the result of concentrating bioclastic remains through winnowingduring interstadials. These units are formed by relatively high current speeds and often represent hiatuses. An indication of a fluctuating current regime is correlativewith the faunal assemblage recorded in the sediment, with corals dominating interstadial/interglacial units and suspended sediment tolerant bryozoans occurring in the finer grained sections of the core. The different facies units are stacked and repeated; indicating that Propeller Mound has been able to continuously re-establish itself as an active mound community, despite the influence of a fluctuating current regime during the last 1.5 Ma. Calculated sediment accumulation rates of 1.12 cm/ky for the cored section and 7 cm/ky for the un-penetrated mound section have implications for previously proposed mound growth models. The values indicate that a very rapid 'booster stage' must be recorded in the unpenetrated depths of the mound, and that Propeller Mound has entered the ?coral bank? or 'burial' stage with ashift to lower sediment accumulation rates. It is likely that the dominant sediment type preserved in the unpenetrated section is likely to be a wackestone, with fewer hiatuses and occurrences of grain- and packstones than in the cored interval.