Encyclopedia of Marine Geosciences

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| Editors: Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede

Abyssal Plains

  • David VoelkerEmail author
Living reference work entry

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DOI: https://doi.org/10.1007/978-94-007-6644-0_211-2

Keywords

Continental Margin Subduction Zone Oceanic Crust Slope Gradient Photic Zone 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Summary

Abyssal plains are flat areas of the ocean floor in a water depth between 3,500 and 5,000 with a gradient well below 0.1°. They occupy around 28 % of the global seafloor. The thickness of the sediment cover seldom exceeds 1,000 m, and the sediments consist of fine-grained erosional detritus and biogenic particles. The first global map of seafloor physiography, published by Heezen and Tharp in 1977, illustrated that the bottom of the ocean is bordered by continental margins of variable width and hosts large submarine mountain chains, the mid-ocean ridges. It also illustrated that between the continental margins and mid-ocean ridges, there exist vast flat and almost featureless regions, the so-called abyssal plains (Fig. 1). The global seafloor map has been refined in recent years (e.g., by Smith and Sandwell, 1995; Becker et al., 2009), in a way that has allowed to perform quantitative analyses of the global distribution of specific seafloor features (Harris et al., 2014).
Fig. 1

(a) Bathymetric map of the western North Atlantic, showing a broad continental shelf and slope, a prominent abyssal plain (the Hatteras Abyssal Plain), and the Mid-Atlantic Ridge. The map was created with the GeoMapApp (www.geomapapp.org); the underlying bathymetric data were published as Global Multi-Resolution Topography (GMRT) by Ryan et al. (2009). (b) shows a depth profile from the continental shelf offshore Newfoundland to the Mid-Atlantic Ridge (AD). In this profile, the continental platform and the continental slope are clearly depicted. Towards the Mid-Atlantic Ridge, the roughness of the seafloor increases with decreasing thickness of the sediment cover. Stippled lines indicate seafloor bedrock age according to Müller et al. (2008)

Bathymetry

Earth is a sphere with an equatorial radius of 6,378 km and a surface area of ~5.1 × 1014 m2. Roughly 71 % or ~3.58 × 1014 m2 of that surface is covered by oceans. The largest part of the ocean floor is at a depth between 3,500 and 5,000 m (with a mean ocean depth of 3,795 m), in the depth range of abyssal plains. Exceptions from this rule are (1) continental margins and epeiric seas, (2) mid-ocean ridges, (3) seamounts and basaltic plateaus that are shallower, as well as (4) submarine trenches at subduction zones that are deeper. These areas are exceptional because plate collision forces govern the shape of the seafloor (at subduction zone trenches), because they form a transition between continental and oceanic crust (continental margins) or because of atypical structure of the ocean crust (basaltic plateaus and seamounts). Mid-ocean ridges stand out as the crust that is formed at these plate boundaries is young and hot. Cooling of the plate leads to crustal subsidence to abyssal depth within ~12 Ma.

Apart from those areas, the seafloor is relatively monotonous in its depth and also very flat. Those regions that typically stretch from the foot of the continental slopes to the mid-ocean ridges have been classified as a specific physiographic entity and termed abyssal plains. The word abyss comes from the Greek word abyssos “bottomless (pool)” indicating the enormous lightless depths, whereas plain refers to the morphological notion of vast monotonously flat areas. Strictly speaking, the term is a kind of contradiction because a plain cannot be bottomless, but it is meant in the same way as its correspondent “deep-sea plain.”

The depth distribution of the world oceans is shown in Fig. 2 in the form of a hypsometric curve. The curve shows a bimodal distribution with one peak between 0 and 500 m and another broader peak between 3,500 and 6,000 m. This distribution reflects the double nature of the seafloor: around the continents, large areas of the seafloor are actually drowned parts of the continents with a crustal structure that is similar to the crust underlying the continents. The deeper part of the ocean is in contrast underlain by oceanic crust that is formed at mid-ocean ridges in a similar composition on global scale. The lifespan of a portion of oceanic crust from its creation at a spreading center to its destruction at a subduction zone ranges seldom exceeds 120 Ma (Müller et al., 2008). Over that time span, the ocean floor cools, contracts, and sinks, rapid first and slower later (see chapter on “Thermal Subsidence”), and this subsidence controls the depth of the largest part of the seafloor.
Fig. 2

Hypsometric curve of the seafloor of the Atlantic, Pacific, and Indian Oceans. The curve shows the percentage of areas of the seafloor that lie within a certain depth interval and exhibits a bimodal distribution: First maximum lies within the 0–500 m range (continental shelves, blue bar), while the largest portion and second maximum lies at water depths between 3,500 and 6,000 m (oceanic seafloor, red bars). The second maximum, indicated by red bars, also is the typical depth range of abyssal plains. The map is based on the global bathymetric data set of Smith and Sandwell (1995)

Abyssal plains grade into the foot regions of continental margins towards the land masses and into areas of increasing topographical roughness towards mid-ocean ridges (Fig. 1a, b). In order to delimit this physiographic domain, the International Hydrographic Organization (IHO) agreed upon a definition that includes water depth and the variation in relief over a certain radius (e.g., the variation in relief has to be <300 m over a radius of 25 cells of 30 arc sec). Following this definition, Harris et al. (2014) calculated that globally, abyssal plains make up 27.9 % of the seafloor or 19.8 % of the entire surface of the Earth.

The flatness of abyssal plains is exemplified in Fig. 1b. It shows a bathymetric profile of the western North Atlantic Ocean from offshore Newfoundland to the Mid-Atlantic Ridge. The profile across the abyssal plain between points B and C is 1,100 km long. Over this distance, the difference in elevation is 600 m, corresponding to a mean slope gradient of less than 0.04°, similar to shelf areas. In contrast, the continental margins stand out with slope gradients in the range of 1–10° (in places up to 30°). The flanks of the mid-ocean ridges show gradients of 10–20°; some seamounts stand out as steep edifices with slopes as steep as 40°. The profile across the abyssal plain between points B and C is 1,100 km long. Over this distance, the difference in elevation is 600 m, corresponding to a mean slope gradient of less than 0.04°. Figure 3 summarizes this information in the form of a slope gradient map.
Fig. 3

Global distribution of abyssal plains and deep-sea basins

Sedimentation

Abyssal plains receive sediments in the form of erosional detritus of the continents and as shell fragments of planktonic animals and algae that live in the upper water column of the open sea. These components form the main constituents of the typical deep-sea or pelagic sediments in varying relative proportions. The growth of the sediment cover in time (the rate of sedimentation) is generally low over abyssal plains (on the order of some cm growth in thickness/ka), as compared to continental margins where sedimentation rates are generally higher by a factor of 100–1,000. Given such low sedimentation rates, on abyssal plain accumulates a sediment cover of some hundred meters. The mean thickness of the sediment cover of abyssal plains is 450 m (Whittaker et al., 2013).

Two reasons account for the comparatively low sedimentation rate: first, the abyssal plains are largely cut off from the main source of sedimentary particles, the erosion of the continents. A large part of sediments that are transported to the sea in rivers are deposited on the shelf regions of the continents during global high stands of the sea level (when the shelf regions around continents are wide). River-transported sediments surpass this “sediment trap” easily only when rivers directly connect to submarine canyon systems, or when the shelf is narrow, or episodically at sea level low stands or when turbidites are shaken off. Wind transport of very fine-grained erosional detritus (dust) is an important constituent of abyssal plain sedimentation to the leeward side of arid regions with a constant wind regime (such as offshore the Saharan coast of Western Africa (Morocco, Mauretania)). Erosional detritus of larger grain size (gravel) is brought into the oceans at high latitudes released by melting icebergs (dropstones).

Second, the bioproductivity (density of primary production) of the open seas is low in general (with the exception of zones of equatorial upwelling) because of limited nutrient availability. Low bioproductivity results in little and seasonally variable fallout of shell fragments and organic material from the photic zone. In addition, only a fraction of that export of material from the photic zone ever reaches the seafloor, as the major part gets dissolved or recycled in the water column. This is specifically true for calcareous fragments that become chemically unstable in the deep sea. The water depth underneath of which no calcareous material is preserved in marine sediments is called the carbonate compensation depth (CCD).

In short, deep-sea sedimentation is sparse and relatively evenly distributed (no local sources), depending mostly on the thin fallout of particles from the photic zone, the so-called pelagic rain of particles, wind transport of dust, and distal turbidites from higher areas. Over time, this kind of sedimentation blankets the oceanic crust and levels its topography resulting in the very smooth and even abyssal plains. The thickness of sediments that rest on the oceanic basement can be mapped in the form of contour lines (isopachs). Global isopach maps (Whittaker et al., 2013) show that the major trend in sediment thickness is from absent close to the mid-ocean ridges where the oceanic crust is young and the distance to the continental sources of sediments is large to some hundred meters close to the foot of continental slopes, where the oceanic crust is oldest and continents are close. This trend is locally overprinted by other effects such as high bioproductivity belts around the equator and bottom current transport of sediments but general to all oceans (Fig. 4).
Fig. 4

Sediment thickness (isopach) map of the western North Atlantic, based on a global data set of Whittaker et al. (2013). Towards the American continent, the sediment thickness of the abyssal plain exceeds 1,000 m which partly explains the flatness of that region. Towards the Mid-Atlantic Ridge, sediment thickness decreases while the ruggedness of the ocean floor increases. The depth profile of Fig. 1 is superimposed

Life in the Abyssal Plains

New observation systems employed in the last decades give evidence for numerous life forms in the deep sea and on abyssal plains. One of the fundamental conditions for higher life forms, the constant supply of oxygen is provided for by deep ocean currents that are part of the so-called global conveyor belt of ocean currents. Still, life in the abyssal plains meets very specific challenges, because of which sometimes they are termed “ocean deserts,” indicating the low diversity and low density of life forms. First, autotrophic (plant) life is impossible in the lightless depth of the oceans. Therefore practically every life form here depends on the “import” of nutrients from the photic (uppermost) zone of the water column in the form of a rain of particles (dead plants and animals, fecal pellets) from above. Second, this rain is very thin over most of the abyssal plains. Only few ocean surface regions (the so-called upwelling zones) have a high primary production and associated food web to produce a constant export of nutrients to the deep sea that can nourish a dense population of animals on the deep seafloor. Third, this rain is variable in time and space. This is exemplified by whale falls. Dead whale bodies that sink to the bottom of the ocean provide food for years for a very specialized group of bacteria and animals. Yet whale falls are so irregular in time and the cadavers are so small in relation to the vast dimensions of abyssal plains that it is a question yet unresolved, how those animals actually find the prey and what they do “in between” the next lucky strike. Fourth, the abyssal plain is covered by relatively soft mud. This is an unfavorable situation for animals that are attached to the ground, preventing them to settle and build colonies. This fact becomes striking when seamounts are considered. Seamounts are solid rock bodies that rise from the abyssal plains. If abyssal plains are termed deserts in terms of biological activity, then seamounts are the oasis.

Cross-References

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Copyright information

© Springer Science+Business Media Dordrecht 2014

Authors and Affiliations

  1. 1.MARUM – Zentrum für Marine Umweltwissenschaften der Universität BremenBremenGermany