An aquatic sill (or an oceanic sill) is a

sea floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

barrier of relatively shallow depth (tens to hundreds of meters) that restricts water movement between benthic zones of an

oceanic basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided into basins fol ...

or lake bottom. There are roughly 400 sills in the Earth's oceans, covering 0.01% of the seafloor. A classic example is the Strait of Gibraltar Gateway between the

Mediterranean sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...

and the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

Formation processes

Aquatic sills are common in

fjord In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Germany, Greenland, the Faroe Islands, Icel ...

s, limiting their water exchange with the ocean. After the last ice age, approximately 18,000 years ago, continental glaciers extended to the

continental shelves A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

and created U-shaped glacial valleys, with long narrow openings that rise upward near the outer shelf, thereby creating sills.

Aquatic sills as barriers

Circulation barriers

Aquatic sills can influence water circulation by restricting the movement of bottom water masses, resulting in partial to total separation of two basins. The restricted water circulation affects temporal variations in

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

and can result in oxygen depletion in deeper water masses. Deep sea corals, Wagner Seamount

Biogeographic barriers

An aquatic sill can be a biogeographic barrier for species between two basins, which commonly include

sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

bryozoans Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

, and cold-water corals. These communities tend to develop in highly productive waters, such as

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...

areas, and build non-tropical reefs, or bioherms, around sills.

Notable examples

The Strait of Gibraltar Gateway

A type example of an aquatic sill is the Strait of Gibraltar Gateway, which connects the

to the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...

, separates the

Iberian Peninsula The Iberian Peninsula (), ** * Aragonese and Occitan: ''Peninsula Iberica'' ** ** * french: Péninsule Ibérique * mwl, Península Eibérica * eu, Iberiar penintsula also known as Iberia, is a peninsula in southwestern Europe, def ...

in Europe from

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to A ...

in Africa, and limits exchange of deep

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is '' flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. ...

between the North Atlantic and Mediterranean sea. The sill (~200 m depth) has elevated seabed topography with two "mounts" (Monte Seco and Monte Tartesos) separated by east-west oriented depressions and north-south elevated morphological structures. At the sill, the eastward Atlantic water inflow meets the deep westward Mediterranean outflow. The sill is characterized by rocky seabed, low, muddy sedimentation, and accumulations of reef-forming cold water corals up to 40-m thick. The gateway formed after the Messinian Crisis, 5.96-5.33 million years ago, when the Mediterranean Sea experienced a near complete drought. During the Zanclean Flood that followed, two deep channels (Canal Norte and Sur) were scoured out. During the late

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

and during the Early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58pull-apart basins under a transtensional regime, thereby forming the gateway. Norway- Fjord on a rainy day

Other examples

Apsheron Threshold The Apsheron Sill, Absheron Sill, Apsheron Ridge or Apsheron Threshold is a major northwest–southeast trending bathymetric high that runs for about 250 km across the whole of the Caspian Sea from Baku in Azerbaijan to the Cheleken Peninsula in Tu ...

Camarinal Sill The Camarinal Sill is the sill separating the Mediterranean Sea and the Atlantic Ocean. This threshold is the shallowest seafloor pass between the Iberian Peninsula and Africa. It is located approximately 25 km west of the narrowest section of ...

Denmark Strait The Denmark Strait () or Greenland Strait ( , 'Greenland Sound') is an oceanic strait between Greenland to its northwest and Iceland to its southeast. The Norwegian island of Jan Mayen lies northeast of the strait. Geography The strait connect ...

Espartel Sill The Spartel or Espartel Sill is one of the sills separating the Mediterranean Sea and the Atlantic Ocean. This threshold is the second shallowest seafloor pass between the Iberian Peninsula and Africa. It is located near the Strait of Gibraltar ...

Mona Passage The Mona Passage ( es, Canal de la Mona) is a strait that separates the islands of Hispaniola and Puerto Rico. The Mona Passage connects the Atlantic Ocean to the Caribbean Sea and is an important shipping route between the Atlantic and the Panam ...

Reference

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