Valerie fig4 nett — In silty sediments, the sediment hole makers thrive. Within a sponge garden area at 230 m depth off the North Norwegian coast (65.5º N) we, for once, observed one of these creatures while slowly coming out ready to defend its home against the Mareano program’s video monster. In this case, the hole maker was a Norway lobster, Nephrops norvegicus, which is common from Iceland to the Mediterranean. Video photo: Mareano

Horseshoe and cold-water coral reefs

Cruise diary: In the framework of this year´s seabed mapping on the Norwegian continental shelf, R/V "G.O. Sars" arrived on Trænabanken last week.

Published: 29.07.2020 Updated: 14.12.2022 Author: Valérie Bellec, Geological Survey of Norway (NGU), Børge Holte, Institute of Marine Research (IMR), Henning Jensen, Liv Plassen and Markus Diesing, NGU

This well-known fishing bank is localized in the Norwegian Sea, southwest off Lofoten, and about 2-3 hours sailing time from the Træna islands in Nordland county. The morphology of the bank is largely uniform, except for a prominent feature named Hesteskoen (Horseshoe), which is located in the eastern part of Trænabanken (Figure 1).

Hesteskoen was formed during the last ice age by the glaciers transporting large sediment masses from the seabed, leaving behind a large hole of about 40 km2. Later the sediment masses detached from the glacier and formed a semi-circular large hill composed of various sediment types, from fine-grained clay to large cobbles and boulders (to learn more about these features, see Rise et al. 2016). Hesteskoen is an example of a landform called a “hill-hole pair”. It is one of many well-known glaciotectonic structures from the ice age on the Norwegian continental shelf.

The top of Hesteskoen hill reaches a depth of 180 m, about 100 m above the surrounding seafloor, while the deepest part of the hole is almost 350 m deep (Figures 1 and 2).

Figure 1. A- Bathymetry data between Tromsø and Trænabanken (200 m grid). The red square shows the location of figure B. B- 50 m grid on Trænabanken where the glacier has transported seabed sediments and deposited them in a horseshoe formation to the right (“hill-hole pair”). High resolution bathymetry data collected by Kartverket/Mareano. Background: GEBCO bathymetry.

The highest part and the sides of Hesteskoen are composed of coarser material than the deeper flat seabed. Gravel, cobbles and boulders, often associated with finer sediments (clay, silt and sand), are quite common in this type of formation seen on video. However, the deepest part of Hesteskoen shows the same soft sediments as the surrounding area (mainly mud and some sand).

Cold-water coral reefs, formed by eye corals (Lophelia pertusa), thrives on hard bottom exposed to bottom ocean currents. This explains why the ongoing Mareano cruise found coral reefs on top of both sides of the elongated Hesteskoen formation where cobbles and boulders are common (Figure 2).

Figure 2. 3D bathymetry showing Hesteskoen with his adjacent hole where the sediments come from (“hill-hole pair”; bathymetry collected by Kartverket/Mareano) and pictures from video lines taken during this cruise in the area. 10 cm between red laser dots. Video photos: Mareano.

Not only corals, but also other seafloor living invertebrates thrive in these specific physical environments which are formed by geological processes. Species that need solid ground and physical support in order to stay upright in the water masses to filter out food particles from the water are relatively common in the hard-bottom locations of Hesteskoen. Borrowing species are living from eating muddy sediments rich in organic material accumulated on the seafloor, like in Hesteskoen’s hole soft sediments. These examples may illustrate how the benthic community composition varies from place to place, thereby indirectly also influences on demersal or pelagic organisms feeding on seafloor living invertebrates.

Kaldtvannskorallrev og uer. — Scene from a cold-water coral reef on the mid Norwegian shelf found last Tuesday at ca. 250 m depth (temperature was ca. 7 degrees C) at Hesteskoen. The picture show eye corals (Lophelia pertusa), bubble gum corals (Paragorgia arborea) and Primnoa corals. Redfish is common in the coral habitats on the Norwegian shelf. Video photo: Mareano

svamphage på Trænabanken — Sponge gardens, consisting of a relatively small number of sponge species, are widespread in the area surveyed by MAREANO; here from the Trænabanken area. Such communities give shelter and food for various other seafloor organisms. Video photo: Mareano

Korallrev med døde koraller — Not only living and healthy corals were observed on the reef habitats found by the Mareano program in the ongoing survey, but also dead eye corals that seem to be outcompeted by new coral growth. Species-rich communities may occur within the coral rubble from dead eye corals (Lophelia pertusa). Video photo: Mareano

sei — At some locations, pollock schools followed us during the 200 m long video transects. Video photo: Mareano

Reference

Rise, L., Bellec, V.K., Ottesen, D., Bøe, R., Thorsnes, T., 2016. Hill-hole pairs on the Norwegian continental shelf. Geological Society, London, Memoirs, 46, 203-204, 2016..