Strong currents shaping the seafloor of Spitsbergenbanken
Cruise diary: When we arrived at Spitsbergenbanken a few days ago, we were met by strong currents. On the seafloor, the currents shape sediments into different types of bedforms, with the most common being ripples, which are centimetres to decimetres high, but also larger bedforms such as sandwaves, or sandbanks which can be more than 10 m high.
Author: Valérie Bellec, Reidulv Bøe, Aivo Lepland and Ingrid Leirvik Olsen, Geological Survey of Norway (NGU), and Børge Holte, Institute of Marine Research (HI)
To form ripples, bottom currents need to be strong enough to move sediment particles such as silt, sand, or gravel. Bottom currents may either be caused by waves (bidirectional, changing direction over a few seconds), tides (bidirectional, changing direction each tidal cycle), or oceanic currents (unidirectional) like the Gulf Stream or the Polar current. Current velocities may be as high as 1 m/s in the shallowest areas of Spitsbergenbanken.
Earlier on this cruise we observed wave action down to 150 m depth on Spitsbergenbanken (LINK). In shallow areas, around 40-50 m depth, currents created by waves are strong enough to move gravel and cause formation of wave ripples. Ripples formed by wave energy are symmetrical, commonly bifurcate, and typically show a flat crest. On Spitsbergenbanken, they are seen as relatively large (often one meter wavelength) and regular undulations on the seafloor. Wave ripples are common features also on beaches.
Ripples formed by unidirectional currents comprise usually of sand, however, silt or gravel ripples also occur. Unlike wave ripples, current ripples show an asymmetrical profile with a gentle stoss-side and a steep lee-side. Depending on the strength of the currents, their crests are straight or undulating. Where currents with different directions meet (e.g., wave currents and tidal currents on top of Spitsbergenbanken) interference ripples with varying crest orientation may form.