KeywordsContinental Margin Subduction Zone Oceanic Crust Slope Gradient Photic Zone
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.”
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.
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).
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.
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