Deep-Sea Sedimentation

Deep-sea sedimentation occurs when particles settle to the ocean floor and material is transported from shallow to deep water. Knowledge of the distribution of sediments and sedimentation processes will help evaluate the potential of the world’s oceans for mining natural resources and storing waste.

Investigation of Deep-Sea Sediments

Sedimentation in the deep sea differs substantially from that in any other environment. Deep-sea sedimentation is pelagic. It typically occurs at great distances from continents and at depths in excess of 1,000 meters (3,281 feet), primarily by the settling of particles through the overlying water. Every 100,000 liters (26,417 gallons) of seawater contains about 1 gram (0.03 ounces) of extremely small particles. Their settling is extremely slow, and sediments accumulate at rates on the order of a few millimeters per thousand years. Transport of material by turbidity currents along the bottom of the ocean from shallow to deep regions is also a mechanism by which deep-sea sediments are deposited, but this is far less important than the settling of particles from the surface. In contrast, current transport dominates sedimentation in all other environments, where sedimentation rates typically exceed several centimeters per year.

The oceans cover 70 percent of Earth, and, as a result, deep-sea sedimentation may be the most common sedimentary process. Early investigations of the seas were limited, however, to coastal processes and oceanic circulation because of the lack of techniques available to study the deep ocean bottom. Knowledge of deep-sea sedimentation was profoundly increased by the voyage of HMS Challenger from 1872 to 1876, which made the first systematic study of the ocean floor. Prior to the Challenger expedition, practical investigations of the oceans concentrated on winds, tides, and water depths in harbors to provide information for commercial navigation and for the laying of submarine telegraph cables. The Challenger crew collected data on the temperature and depth of the oceans, marine plants and animals, and sediments on the ocean bottom. Deep-sea sediments were described by John Murray, naturalist to the expedition. He noted that sediments in the deepest parts of the ocean are fine-grained and are composed of particles that settled from the surface. Comparison of rocks exposed on land with those dredged from the ocean led to the assertion that deep-sea sediments could not be found on land. This conclusion caused great debate, and the interpretation of some rocks as deep-sea deposits by a few scientists persisted despite opposition by most of the scientific community.

Investigation of deep-sea sedimentation flourished again during and immediately after World War II. Despite the voluminous amount of data available by 1950, the prevailing doctrine in the first half of the twentieth century was similar to that in the previous century: The continents and oceans were generally believed to be permanent features of Earth’s surface that had developed to their present form near the beginning of geologic time. Thus, the ocean basins were billions of years old, ocean-bottom sediments were tens of kilometers thick, having accumulated from early in the geological history of Earth to the present, and deep-sea sediments were not exposed on land, a point that was still severely contested. Then, during the 1950s two discoveries shocked the oceanographic community. First, fossils taken from seamounts (submarine mountains) in the Pacific Ocean at depths of 2 kilometers (1 1/4 miles) were only 100 million years old, suggesting that subsidence of the ocean floor was recent. Second, deep-sea sediments were found to be less than 200 meters (656 feet) thick. The advent of the theory of plate tectonics in the 1960s profoundly changed ideas about the ocean basins by providing both an explanation for the two observations of the previous decade and a mechanism for the emplacement of deep-sea sediments onto continents. In addition, the motion and cooling of the oceanic plates predicted by plate tectonics explain the global distribution of deep-sea deposits.

Pelagic Sediments

Pelagic sediments are characterized by their fine grain size. Few particles are larger than 0.025 millimeters, and most are smaller than 0.001 millimeters. Particles include terrigenous debris (derived from the continents), such as clay minerals, quartz, and feldspar; volcanogenic grains (derived from volcanoes), such as volcanic glass, pumice, and ash; biogenic material (derived from living organisms), such as fecal pellets (waste produced by organisms inhabiting the surface waters) and skeletons of planktonic plants and animals; and cosmogenic matter (derived from space or the cosmos), such as “space dust” and pieces of meteorites. Pelagic sediments that contain more than 30 percent biogenic material are called oozes. If the biogenic debris is composed of calcium carbonate, the sediment is a calcareous ooze. If the biogenic debris is composed of silica, the sediment is a siliceous ooze. Most calcareous oozes consist of foraminiferans, which are single-celled animals that secrete calcium carbonate and live in the surface waters. In contrast, siliceous oozes contain diatoms (green, unicellular algae) in cold water and radiolarians (silica-secreting single-celled animals) in warm water. Pelagic clays are sediments that contain less than 30 percent biogenic debris.

The two major controls on pelagic sedimentation are the calcium compensation depth (CCD) and the fertility of the surface waters. The CCD reflects the interplay between the release of carbon dioxide during the decay of surface organisms and the dissolution of the calcium carbonate skeletons as they descend through the ocean. Above the CCD, calcareous oozes are dominant, whereas below the CCD, siliceous oozes and pelagic clays are common. For example, in one region of the ocean, sediments decreased from 90 percent calcium carbonate at a depth of 1,000 meters (3,281 feet) to 20 percent calcium carbonate at a depth of 5,000 meters (16,404 feet). Oozes accumulate below areas of high fertility where upwelling brings nutrient-rich bottom water to the surface. Because upwelling creates increased productivity of both calcareous and siliceous organisms, siliceous oozes form only where the water depth exceeds the CCD. Calcareous oozes, the most abundant biogenic sediments in the ocean, cover only 15 percent of the Pacific Ocean bottom but blanket 60 percent of the Atlantic Ocean floor. This difference reflects the combination of a shallower CCD and greater water depth in the Pacific Ocean than in the Atlantic Ocean. Siliceous oozes are in areas of elevated CCDs such as the equatorial regions, the subarctic and subantarctic zones, and the continental margins of northwestern South America and eastern Africa.

Sedimentation of pelagic clays is restricted to the central portions of the oceans where the fertility is low. This reflects the difference in settling rates between nonbiogenic particles and fecal pellets. Because of its small size, a particle may take many years to sink through the ocean. The concentration of many particles into fecal pellets by organisms feeding at the surface, therefore, is thought to be an important mechanism by which debris settles to the bottom. The greater size of the pellets allows them to fall much faster than unconsolidated particles. It is only in areas of low productivity, therefore, where biogenic material does not overwhelm the sediment. Wind is the primary means by which nonbiogenic particles reach the sea. Dust is carried in the atmosphere at great distances from continents and constitutes as much as 10 percent of pelagic sediment.

Other Processes

Additional processes in deep-sea sedimentation include the rafting of glacial debris by ice, the transport of terrigenous and shallow-water material by turbidity currents along the ocean floor, and the precipitation of nodules on the ocean bottom. Pieces of ice calve off glaciers in the polar regions and float toward warmer waters where they eventually melt, releasing glacial sediment into the ocean. Although this process is most common in the south polar region today, glacial sediments greater than ten thousand years old elsewhere on the ocean floor suggest the process extended over a much greater area in the past. Turbidity currents generated along continental slopes may traverse hundreds of kilometers of ocean basin where ridges and depressions do not occur. Far away from continents, the currents commonly contain only silt but may deposit the grains over great distances. These currents also may locally erode the sea floor, creating breaks in the continuity of the sediment pile by removing the most recent sediments and exposing older sediments at the abraded surface. Chemical reactions of seawater with the surface of the sediment cause the precipitation of ferromanganese (composed of iron and manganese) nodules in areas where sediments accumulate slowly. The nodules form by nucleating around a manganese source in the sediment and by accreting minerals from the seawater. Such nodules may cover more than 40 percent of the sea floor.

Sedimentation in the deep sea is a complex interaction between water depth, fertility of the surface waters, and current transport. To illustrate how the depth dependence of sedimentation affects the global distribution of sediments, one must understand the relationship between plate tectonics and the topography of the ocean floor. At a midocean ridge, which rises above the surrounding ocean basin, molten rock is extruded to form a new sea floor. Calcareous oozes accumulate on the new sea floor because of its depth above the CCD. As more sea floor forms, the older sea floor moves away from the midocean ridge toward the deep ocean basin (seafloor spreading), sinking to lower depths as it does so. The old sea floor gradually descends below the CCD, where siliceous oozes accumulate above the calcareous oozes. Eventually, the motion of the sea floor carries it below a region of low surface productivity such that pelagic clays are deposited above the siliceous oozes. This vertical sequence has been documented in several localities throughout the world’s oceans.

Study of Deep-Sea Sedimentation

Techniques used to study sedimentation in the deep ocean are diverse and include piston coring, seismic reflection profiling, the use of deep-diving submarines, and experiments performed both at sea and in the laboratory. Piston coring is the most important technique because it enables direct sampling of the ocean floor. It uses the same kind of rotary-drilling equipment that is standard in petroleum exploration to drill into the top few hundred meters of the sea floor. The cores are 7 centimeters (2 3/4 inches) in diameter and provide samples that are large enough to preserve features that yield insights into the processes by which the sediment accumulated. For example, current-produced features in some samples were the first direct evidence of currents in the deep seas. In addition, piston coring confirmed the vertical sequence of sediments predicted by plate tectonics.

To map the ocean floor using a 7-centimeter (2 3/4-inch) drill core is time-consuming and inefficient. Seismic reflection profiling has the advantage of providing information about the thickness and distribution of layers in the sediments on the sea floor relatively quickly. The two major disadvantages are that it cannot determine the composition of the sediment and that it identifies only layers that are tens of meters thick. In seismic reflection profiling, a low-frequency sound source is towed over the bottom. The sound waves penetrate and are reflected by the various layers within the sea floor to several kilometers below the boundary between the sediment and the water. The returning signals are displayed on strip charts that reveal the water depth and the thickness and pattern of layers in the sediments. One of the most significant early contributions of seismic reflection profiling was the evidence that sediments on the ocean floor were too thin to have been accumulating since early in Earth’s history, an observation explained by plate tectonics.

The use of deep-diving submarines has allowed scientists to observe the ocean floor directly. Previously, the ocean floor was only photographed. Deep-diving submarines allowed scientists to confirm or reject hypotheses based on other indirect techniques. The ranges and endurance times of the submersible vessels, however, are limited. In addition, they frequently cannot be used in areas where current velocities are greater than 1 meter (3 feet) per second.

Simple experiments either at sea or in the laboratory also can be illuminating. For example, the existence of the CCD was verified by lowering spheres of calcium carbonate to various water depths in the Pacific Ocean. When the spheres were examined, it was discovered that the spheres in the deepest water had dissolved the most. Scientists in the laboratory conduct experiments on settling rates by dropping particles of various sizes in large beakers and measuring the time necessary for the particles to reach the bottom. Thus the amount of time represented by the thickness of sediment on the ocean floor was roughly estimated at a few hundred million years.

One of the most important developments in deep-sea sedimentation was the creation in 1965 of the Deep Sea Drilling Project (DSDP), now known as the Ocean Drilling Project (ODP), to drill and investigate the ocean floor. The project began in earnest in 1968 with the maiden voyage of the American ship the Glomar Challenger, which was outfitted with state-of-the-art navigational, positioning, and drilling equipment. International teams of scientists boarded the ship in ports all over the world and remained at sea for two months. Since its inception, ODP has undertaken more than sixty-five voyages and contributed significantly to scientists’ understanding of the ocean.

Significance

The oceans cover nearly three-fourths of the surface of Earth.  However, understanding deep-sea sedimentation is important for several other reasons. First, sediments on the ocean floor yield insight into bottom currents and water depths, which affect the navigation of submarines and the installation of underwater communications cables. Second, the sediments preserve the climatic record of the past few hundred thousands of years. For example, glacial deposits approximately ten thousand years old are found over a much greater area of the bottom of the ocean than recent glacial deposits, suggesting that transport of glacial debris by ice was very common ten thousand years ago. This implies that the temperature of Earth’s environment was lower in the past, allowing the formation of significant quantities of ice. Scientists can use this information to gauge present-day climate patterns and predict future fluctuations in the global climate. Third, the seas receive a significant amount of the garbage produced by modern society. Knowledge of sedimentation rates and currents in the deep sea helps to establish the length of time necessary to bury the waste, the direction the waste will travel prior to its burial, and the effect of dumping waste on the overall health of the ocean.

Fourth and most importantly, understanding deep-sea sedimentation and the global distribution of deep-sea sediments provides a framework within which one can estimate the economic potential of the deep sea and minimize the damage from ocean exploitation. As the reserve of natural resources on the continents dwindles because of expanding demand from high-technology industries, exploding population growth, and increasing consumer appetite, attention focuses on the oceans as a possible source of raw materials. Important resources that may be found locally in the deep sea include oil and gas. The decay and accumulation of plants and animals are essential to the generation of hydrocarbons and are processes that occur over a large area of the sea floor. Emphasis also is placed on mining the deep sea for important materials such as manganese, gold, cadmium, copper, and nickel. Similar to most new industries, large-scale exploitation of the natural resources of the deep sea is limited at present by the current state of technology. The incentive to mine the oceans, however, will grow as cheaper and more sophisticated technologies are created.

Principal Terms

calcareous ooze: sediment in which more than 30 percent of the particles are the remains of plants and animals composed of calcium carbonate

carbonate compensation depth (CCD): the depth in the oceans at which the rate of supply of calcium carbonate equals the rate of dissolution of calcium carbonate

clay: a sediment particle less than 0.1 millimeters in diameter

clay minerals: a group of hydrous silicate minerals characterized by a structure of layers of thin sheets that are held together loosely

pelagic: “of the deep sea”; refers to sediments that are fine-grained and are deposited very slowly at great distances from continents

siliceous ooze: fine-grained sediment in which more than 30 percent of the particles are organic remains of plants and animals composed of silica

turbidity current: a mass of water and sediment that flows downhill along the bottom of a body of water because it is denser than the surrounding water; common on continental slopes

upwelling: an ocean phenomenon in which warm surface waters are pushed away from the coast and are replaced by cold waters that carry more nutrients up from depth

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