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.
Published: 05.10.2021Author: 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.
Wave ripples
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.
Wave ripples have parallel crests on bathymetry maps and show a quite regular pattern. They mostly occur in shallow areas, as they need strong wave currents to form. Here, the wave ripples are 15-20 cm high and a few hundred meters long. Bathymetry data from MAREANO/KartverketWave ripples comprising mainly of sand and gravel size shell fragments, with a typically symmetrical flat crest. Red laser dots are 10 cm apart. Wave ripple comprising of sand and gravel size rock particles and shell fragments. Some animals, such as the sea cucumber (Cucumaria), thrive in strong currents and can be ubiquitously found between wave ripples. Red laser dots are 10 cm apart.Wave ripples comprising of gravel (in the foreground). In the troughs between the wave ripples (light grey areas) we find sand ripples migrating at 90 degrees to the gravel ripples. Picture from low resolution overview camera with feet of the video rig to the left and to the right.Wave ripples trending from the left to the right. Picture from low resolution overview camera.
Current ripples
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.
Current ripples are typically irregular, showing various shapes and dimensions. The upper panel shows large ripples on a 10 m high sandbank, whereas the lower figure shows interference ripples and unidirectional current ripples on top of two crossing sandbanks. The sandbanks are up to 10 m high. Note the parallel wave ripples at the foot of the sandbank. The depth varies form 25 m (red) to 42 m (violet) in the upper panel, and 32 m (red) to 45 m (violet) in the lower panel. Bathymetry data from MAREANO/KartverketClose-up of 10 cm high current ripples migrating towards the lower left. Here the current is low enough to allow animals to settle on the seafloor. Red laser dots are 10 cm apart.Sand ripples, 2-3 cm high formed by strong unidirectional current. Hydrozoans, anchored to gravel, cobbles, boulders and shells are dragged by the current down towards the left. The distance between the red laser dots is 10 cm.Sand ripples, 5 cm high. On a seabed like this, with migrating sand ripples, there is no solid ground for the fauna to stick to. A hydrozoan is captured in the current, rolling along the seabed. The distance between the red laser dots is 10 cm.Ripples on a 10 m high sand ridge (sandbank). The current is very strong, and particles are jumping along the seabed and suspended in the water above the seabed. Few animals can live on this rapidly changing seabed, except for some species of polychaeta. The distance between the red laser dots is 10 cm.
