An aquatic sill (or an oceanic sill) is a sea floor barrier of relatively shallow depth (tens to hundreds of meters) that restricts water movement between

benthic zone The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

s of an oceanic basin 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 east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...

and the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

Formation processes

Aquatic sills are common in fjords, limiting their water exchange with the ocean. After the last ice age, approximately 18,000 years ago, continental glaciers extended to the continental shelves 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.

Biogeographic barriers

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

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

ns, bivalves, 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 sur ...

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

Notable examples

The Strait of Gibraltar Gateway

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

to the

, separates the

Iberian Peninsula The Iberian Peninsula ( ), also known as Iberia, is a peninsula in south-western Europe. Mostly separated from the rest of the European landmass by the Pyrenees, it includes the territories of peninsular Spain and Continental Portugal, comprisin ...

in Europe from

Morocco Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...

in Africa, and limits exchange of deep

fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

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 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 epoch (geology), 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 mea ...

and early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58pull-apart basins under a transtensional regime, thereby forming the gateway. Norway- Fjord on a rainy day

Other examples

* Apsheron Threshold * Camarinal Sill * Denmark Strait * Espartel Sill * Mona Passage

References

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