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Xavier Monteys, Geological Survey of Ireland (Ireland)
Daniel Praeg, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) (Italy)
Silvia Caloca, Geological Survey of Ireland (Ireland)
Soledad Garcia-Gil, University of Vigo (Spain)
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Sinuous furrows and sub-circular craters interpreted as relict iceberg keel marks are examined across the Porcupine and Irish Rockall Banks, NE Atlantic, using Irish National Seabed Survey geophysical datasets (multibeam data and 3.5 kHz subbottom profiles). The keel marks are observed over a total area of c. 40,000 km2 of the two banks, in water depths (WD) of c. 180-600 m; over 40,000 individual seabed features have been mapped. The widespread seafloor features record the keels of floating icebergs up to 450-500 m in draft that calved into the Atlantic Ocean from the last European and/or North American ice sheets. On the Porcupine Bank, iceberg furrows and craters are observed over an area of c. 34,000 km2, to a local maximum depth of 580 m; they are present on about 20% of the seabed between 200-500 m WD. Furrows are sinuous features that vary from 4-15 m deep, 50-500 m wide and from hundreds of meters to over 20 km long (mean = 950 m). Some larger furrows have lateral berms 1-2 m high. Sub-circular craters can be up to 1 km in diameter (mean = 230 m) and up to 20 m deep, with surrounding berms up to 4 m high. Craters at the end of furrows are interpreted as terminal grounding pits. The furrows cover a broad spectrum of orientations, but over 1/3 lie in the range 270-310 (NNW- SSE); where the direction of scour can be inferred from the terminal pit, the most common orientation is NNW. On the Rockall Bank, keel marks are observed over an area of c. 6000 km2, to maximum water depths of 500 m in the south and 600 m in the west; they are present on about 10% of the seabed between 200-500 m WD. Furrows on the Rockall Bank are generally smaller (1-6 m deep), possibly due to different sediment properties. On both the Porcupine and Rockall Banks, above c. 225 m WD the keel marks become shallower and shorter and the number of craters increases; they are not recognized above c. 180 m WD. Global sea level was up to 140 m lower at the last glacial maximum (LGM), so that the top of the Porcupine Bank (150 m WD) would have been near sea level and a larger area of the Rockall Bank would have been exposed. The lack of iceberg keel marks above present depths of c. 200 m may be due to smaller icebergs scouring harder seabed, as suggested by 3.5 kHz data and samples, and/or erosion by currents. The distribution and orientations of the keel marks at greater depths are inferred to reflect the influence of ocean currents similar to those of the present moving along the flanks of the banks. On the Porcupine Bank, the highest density of marks is in the southwest in 300-400 m WD, extending down to 550 m WD; in contrast, there are no keel marks deeper than 400 m to the east in the Porcupine Seabight. This suggests that most larger icebergs followed paths determined by north-flowing deep-water currents crossing the mouth of the Seabight from the south, whereas some smaller icebergs were able to enter the Seabight to be carried north by currents along its upper slopes.
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