World Ocean Census - Extract 19 - Abyssal plains

3 June 2010

Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor. They are among Earth’s flattest and smoothest regions – and the least explored. Abyssal plains cover the vast majority of the ocean floor.

Generally lying between the foot of a continental rise and a mid-oceanic ridge, they result when an uneven surface of oceanic crust (made of basalt) is covered by fine-grained sediment, mainly clay and silt. Much of this sediment is deposited by currents that have traveled down the continental margins, along submarine canyons (steep-sided valleys on the seafloor) and into deeper water. The remainder of the sediment is chiefly dust (clay particles) blown out to sea from land and the remains of small marine plants and animals (plankton), which sink from the upper layer of the ocean. The sediment deposition rate in remote ocean areas is relatively slow, estimated at 2 to 3 centimeters (about 1 inch) per thousand years. In some areas of these vast plains, manganese nodules are common, containing significant concentrations of metals that include iron, nickel, cobalt and copper. These nodules may prove to be a significant resource for future mining. Sediment-covered abyssal plains are less common in the Pacific Ocean than in other major ocean basins, because sediments carried by currents are trapped in submarine trenches that border the Pacific.

Because these wide expanses of ocean bottom are very difficult to reach, questions about how many species are there and how they are distributed have long remained unanswered. The Census of Marine Life has been the first large-scale effort to find some of these answers. Census scientists have collected hundreds of new species, and nearly 200 have already been described and named. Identification and description of deep-sea organisms is very important because, in any sample taken at any spot in the deep sea, at least half of all animals have never been seen before. At this time, estimates of the number of species living in the deep sea vary greatly from 500,000 to 10 million; such a wide range indicates the poor state of our knowledge. Before the Census began, scientists had to rely on a few football fields’ worth of sampled seafloor to make assumptions about half of Earth’s surface!

Text and images reprinted with permission from World Ocean Census: A Global Survey of Marine Life by Darlene Trew Crist, Gail Scowcroft and James M. Harding Jr., Firefly Books, 2009

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