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Cobbles and boulders with some gravel in between. Here the stones are coated by the encrusting algae (Lithothamnion sp.), giving them a vivid purple colour. 10 cm between red dots. (Photo: MAREANO/HI)

Shallow, but beautiful

MAREANO is currently on cruise gathering visual and physical seabed samples from several areas around the Svalbard archipelago in the Norwegian Arctic. At the moment we are back in outer Kongsfjordrenna trough, offshore of northwest Svalbard, after a couple of days working closer to shore in Kongsfjorden due to poor weather. Here we tell you a little bit about what we have seen here.

Beautiful Kongsfjorden where we found shelter during the strong wind period.
Beautiful Kongsfjorden where we found shelter during the strong wind period.

After seeking shelter in beautiful Kongsfjorden during a spell of bad weather, we returned out to the open sea. Some of the transects that we acquired video data along were located on a shallow bank, only 30 m depth at the shallowest.

Bathymetry map in and outside Kongsfjorden. The black/white circles indicate the location of the video transects (Figures 3 and 4). Left upper panel: backscatter map (red square in the bathymetry map) showing lower values in sandy areas and higher values in the cobbles and boulders area. (Map: MAREANO/NGU. Bathymetry: MAREANO/Kartverket. Sentinel satellite imagery: Courtesy of Norwegian Polar Institute)
Bathymetry map in and outside Kongsfjorden. The black/white circles indicate the location of the video transects (Figures 3 and 4). Left upper panel: backscatter map (red square in the bathymetry map) showing lower values in sandy areas and higher values in the cobbles and boulders area. (Map: MAREANO/NGU. Bathymetry: MAREANO/Kartverket. Sentinel satellite imagery: Courtesy of Norwegian Polar Institute)

Such shallow depths are prone to strong currents and waves can easily erode the seafloor, meaning that mostly all fine-grained sediments are washed away, leaving only a cover of very coarse sediments (gravel, cobbles and boulders; see photos below). The light can also reach the seafloor, allowing algae to thrive on the stones.

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Cobbles and boulders with some gravel in between. Here the stones are coated by the encrusting algae (Lithothamnion sp.), giving them a vivid purple colour. 10 cm between red dots. (Photo: MAREANO/HI)

As we move just a little bit deeper, we start seeing small depressions where gravel has accumulated on top of cobbles and boulders. In some of these areas gravel ripples have formed. These large symmetric ripples form in a similar way as sand ripples (see "Rare dyr i strie strømmer" for more information on how ripples form) and are quite common in shallow areas along the Norwegian coast. Symmetric ripples area usually formed by waves, whereas asymmetric ripples are formed by unidirectional currents.

Accumulation of gravel forming small gravel ripples. 10 cm between red dots. (Photo: MAREANO/HI)
Accumulation of gravel forming small gravel ripples. 10 cm between red dots. (Photo: MAREANO/HI)

It is common to see a clear gradient from coarser to finer sediments when moving into deeper and calmer areas. This reflects the decrease in current strength, and the sediments are often well sorted (by grain size). We see this here, a little deeper, where the currents are slightly weaker. This allows sand deposition in small basins. Nevertheless, the currents here are strong enough to form asymmetric ripples, where the steep slope (lee side) points down-current.

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First picture: Sand accumulation in small basins. Second picture: Strong currents allow sand ripples to form. 10 cm between red dots. (Photo: MAREANO/HI)

Gravel and sand ripples are a clear indicator of active sediment transport along the seabed. The shape of the ripples can reveal whether the movement is caused by ocean currents or by the impact of the waves touching the bottom. Either way, their presence indicates that there is a lot of energy in the environment. Animals that live in this type of habitat must be able to “take a beating” (see "Rare dyr i strie strømmer" for more information on how ripples form and life in a high-energy environment).