Loki's Castle

Loki's Castle is a field of five active hydrothermal vents in the mid-Atlantic Ocean, located at 73 degrees north on the Mid-Atlantic Ridge between Iceland and Svalbard at a depth of 2352 m. When they were discovered in mid-July 2008, they were the most northerly black smoker vents.

They are of geological interest as they occur in a relatively stable region of the Earth's crust with diminished tectonic forces and consequently fewer hydrothermal vents. They are the host site of a biologically distinct archaea, the Lokiarchaeota.

Discovery
The vents were discovered by a 25-person multinational scientific expedition of the University of Bergen, Norway, more than 120 nmi north of what were previously the northernmost known, discovered in 2005. The 2005 and 2008 expeditions were both led by geologist Rolf Pedersen of the university's Centre for Geobiology, aboard the research vessel G.O. Sars (named after the Norwegian marine biologist Georg Ossian Sars and launched in May 2003 ). The vents were located using a remotely controlled underwater vehicle.

The vent field was given the name Loki's Castle as its shape reminded its discoverers of a fantasy castle. The reference is to the ancient Norse god of trickery, Loki. It was felt to be "an appropriate name for a field that was so difficult to locate".

Activity
The five active chimneys of Loki's Castle are venting water as hot as 320 C and sit on a vast mound of sulfide minerals that is about 825 ft in diameter at its base and some 90 m across its top. A member of the 2008 expedition, oceanographer Marvin Lilley, has speculated that this may be the largest such deposit ever seen on the sea floor. The active chimneys are mostly black in colour but are covered with mats of white bacteria that are living on minerals and materials emitted by the vents. The older chimneys are mottled red in colour due to the presence of deposits of oxidised iron.

2 m gravity cores were collected from the vent field in 2010.

Geology
Loki's Castle is located on the Gakkel Ridge (previously the Arctic Mid-Ocean Ridge), where the Mohns and Knipovich ridges converge. Ocean core complexes are found to the northwest.

Loki's Castle is described in literature as a sediment-influenced basalt-hosted hydrothermal field. Due to boron isotopic composition of hydrothermal fluids, it is suggested that vent fluid is recharged (or percholates into the seafloor) in regions densely concentrated with oceanic sediments, rather than unsedimented igneous rock of ridge flanks.

21 types of minerals have been identified at the vent field. The chimneys are predominantly sphalerite, pyrite, and pyrrhotite, and small amounts of chalcopyrite. Small barite chimneys are in the vicinity of the main field, where venting is less pronounced.

Fluid chemistry
Loki's Castle fluids are rich in volatile gases, namely hydrogen sulfide, hydrogen, and methane.

Ecology
Loki's Castle has dense mats of bacteria that use the minerals and compounds expelled by the vents. Preliminary observations have indicated that the warm area around the vents of Loki's Castle is a marine ecotope populated with apparently unique and diverse microorganisms, unlike other observed marine hydrothermal vent ecosystems. One of these, an archaeal phylum named Lokiarchaeota, was discovered and named after Loki's Castle. Lokiarchaeota is renowned as a potential linkage between prokaryotes and eukaryotes.

The bristleworm Nicomache lokii (Maldanidae) is thought to be a key species in the fauna surrounding the hydrothermal vents in the area. This species is one of the more than ten species that were newly discovered here.



Sclerolinum contortum tube worms are located at the field on the largest sulfide mounds. Melitidae amphipods are found amongst the tube worms and are common in chimney crevices.

Metagenome analyses by Bäckström et al. in 2019 showed that there must be a whole series of previously unknown viruses that are known as LCVs or Loki's Castle Viruses. These are primarily giant viruses of the Megaviricetes class in the phylum Nucleocytoviricota (NCLDV) in the area of Loki's Castle. Of the 23 high-quality NCLDV genomic bins:


 * 15 are related to pithoviruses,
 * 5 are related to Marseille viruses,
 * 1 are related to iridoviruses, and
 * 2 are related to klosneuviruses

The iridovirus-like gene sequence has been named "LCIVAC01".