3.4: Seafloor Sediments

The seafloor is covered by sediment ranging in thickness from zero on a small fraction of the ocean floor to several kilometers. As Chapter 6 discusses, many secrets of the Earth’s history are to be found in these sediments. Because the sediments slowly accumulate layer upon layer, history is preserved in sequence; older sediment is found at progressively greater depths below the seafloor. The upper few to tens of centimeters of sediment are especially important because many living organisms inhabit these sediments, and because the processes that affect the fate of chemicals in and on sediment particles are also concentrated in this zone. Therefore, oceanographers are interested in obtaining two basic types of sediment samples:

• Samples that contain an undisturbed sequence of the layers of sediment from the sediment surface down as far as necessary to cover a long period of history

• Large samples of the top few tens of centimeters of sediment, within which most non-microscopic organisms live

Aside from samples taken in very shallow water, the earliest sediment samples retrieved from the oceans were those that adhered to the lump of tallow at the end of a sounding line. Such samples were useful only for a gross characterization of sediment color and the size and type of the sediment grains. When he explored Baffin Bay in search of a Northwest Passage in 1820, Sir John Ross had his blacksmith construct a “deep-sea clam.” That device, the forerunner of grab samplers used today, collected several kilograms of greenish mud containing living worms and other animals from water depths of almost 2000 m.

Grab Samplers and Box Corers

In its basic design, a grab sampler consists of a sealed metal container, usually with two halves that open at a top hinge like a clamshell (Fig. 3-7a). The sampler is lowered with the clamshell jaws open. When it hits the bottom, it sinks into the sediments. As it is pulled back out of the sediment a mechanism causes its two halves to close. It is then retrieved with its sediment sample within the clamshell, thus grabbing a sample of sediment.

Figure 3-7. Methods of sampling near-surface seafloor sediments. (a) Grab samplers are used to obtain large samples of surface sediments. (b) Box corers are used to collect large-volume undisturbed samples of the upper few centimeters of sediment. The box corer is forced into the sediments by its weight and is then closed off by a spade arm that cuts through the sediment and under the box.

Grab samplers are relatively light and simple to operate, but they can disturb and partially mix the sediment they retrieve. To minimize disturbance, box corers often are used. A box corer consists of a supporting framework that is lowered to sit on the seafloor, a heavily weighted box with an open bottom that sinks about 20 to 30 cm into the sediment, and a blade that slices under the box from the side to hold the sediment in the box when the frame is retrieved (Fig. 3-7b). Because they have a wide opening, box corers can collect large amounts of sediment, but they often weigh several hundred kilograms making them difficult to deploy from research vessels.

Gravity and Piston Corers

To take deeper sediment samples, corers are used. Corers consist of a tube that is open at the bottom end like an apple corer. The tube is forced vertically into the sediment. When the tube is pulled out of the sediment, the sediment core is usually held inside by a core catcher (Fig. 3-8). There are two basic types of corers: the gravity corer and the piston corer. Gravity corers are allowed to fall freely on the end of a cable; they strike the bottom with great force and are driven into the sediment by weights mounted at the top of the core tube (Fig. 3-8a). Piston corers, and sometimes gravity corers, are attached to a release mechanism at the end of a cable (Fig. 3-8b). In a piston corer, the action of the piston helps the core slide into the core barrel so that longer cores can be obtained, and it helps minimize vertical distortion and disturbance of the core’s sediment layers.

Figure 3-8. Sediment corers are used to obtain long, mostly undisturbed, vertical samples of soft seafloor sediments. Most corers have a core catcher that consists of a ring of flexible metal leaves around the inside of the core tip. The leaves are easily pushed aside as the corer enters the sediments, but they are bent backward to close off the tube as the corer is pulled out. (a) Gravity corers are driven into the sediment by their weight and can obtain cores up to about 4 m long. (b) Piston corers are usually suspended from a release arm, which is held in the closed position by a trigger weight that hangs from the end of the trigger arm to below the corer itself. When the trigger weight hits the bottom, the trigger arm is released, allowing the weighted corer to fall freely into the sediments. When the tip of the core barrel reaches the seafloor, a loop of cable between the trigger assembly and a piston inside the core barrel becomes taut holding the piston in position. The movement of the piston causes a small reduction of the water pressure in the barrel above the sediment, helping the sediment core slide into the barrel. The piston mechanism increases core length to longer than 50 m depending on sediment type and minimizes vertical distortion of the sediment layers.

Most sediment cores are taken in muddy sediment, but gravity and piston corers cannot penetrate very sandy or some other types of sediment. In these situations, special corers are needed to force the core barrel into the sediment, either by vibrating it mechanically or by forcing air or water down the outside of the barrel to blow the sediment away.

The smallest corers weigh a few tens of kilograms and take short cores (up to about half a meter long); the largest weigh several tonnes and can take cores more than 50 m long. Deploying and retrieving one of the largest corers is an exacting task that is now performed by computer driven mechanical systems on specially equipped research vessels.

Unfortunately, and to the frustration of researchers who may have waited hours for the sampler to be lowered and retrieved, especially in deep water, grab samplers and corers often fail to penetrate the sediment or close properly. There are several reasons that these failures occur. For example, if currents are strong and deflect the wire from vertical, the sampler might impact the seafloor at too great an angle. In other cases, the sampled material might jam in the closure mechanism. Even a tiny opening in the closure mechanism can cause the entire sample to be lost as the sampler is hauled back through the water column.

Drilling Ships

To study older layers, scientists must explore deeper in the ocean sediment than can be reached by corers. In addition, the bedrock beneath the ocean sediment holds valuable clues to the history of the Earth and its oceans. In 1968, the U.S. began using a unique drilling ship, the Glomar Challenger, to sample the deeper sediment layers and rocks. The ship was capable of drilling up to about 2000 m long sediment cores in water as deep as 6 km. The Deep Sea Drilling Program (DSDP) used the vessel to obtain more than a thousand cores from throughout the world ocean. These core samples helped to confirm the theories of seafloor spreading and continental drift (Chap. 4).

In 1983, the DSDP was succeeded by the Ocean Drilling Program (ODP), an international cooperative program funded jointly by the U.S., Canada, West Germany, France, Japan, the United Kingdom, Australia, and, at one time, the Soviet Union. In 1985, the more sophisticated JOIDES Resolution replaced the Glomar Challenger. In 1990, in a water depth of 5700 m, 2500 km south of Japan, the JOIDES Resolution drilled through 200 m of recently formed volcanic rock and 460 m of sediment lying below it. A sample of sediment was retrieved that was estimated to be 170 million years old and is believed to be the oldest remaining ocean floor sediment, except for that found on small fragments of tectonic plates that have avoided subduction.

In 2003, the ODP was succeeded by the Integrated Ocean Drilling Program and then the Integrated Ocean Discovery Program (IODP), led jointly by the U.S. and Japan. This program operated two drilling ships, the JOIDES Resolution and a newer Japanese vessel called Chikyu. The Chikyu can drill holes as deep as 6 km beneath the seafloor, far surpassing the 2-km limit of the JOIDES Resolution. The Chikyu is also able to drill in shallower water, and it has the ability to prevent blowouts (uncontrolled releases of gas or oil) if it penetrates formations that contain oil and gas. This capability allows Chikyu to drill in many locations where the JOIDES Resolution could not. However, in 2023, JOIDES Resolution was able to recover the first ever core sample of Earth’s mantle material while drilling on the Mid-Atlantic Ridge. The IODP program ended, and the JOIDES Resolution was decommissioned in 2024. Ocean drilling is now performed by Japan’s Chikyu but a new drill ship, China’s Meng Xiang is scheduled to begin operation in 2025 and is expected to be able to drill deeper into the mantle.

Seismic, Magnetic, and Gravity Studies

Because obtaining sediment core or drill-core samples is time-consuming and expensive, oceanographers can only sample a few locations directly. Fortunately, certain remote sensing techniques can provide information about the sediment in areas where actual seafloor samples are unavailable. Seismic profiling and measuring magnetic and gravitational fields are the principal techniques for obtaining such information.

Like sonar measurements, seismic profiling uses a sound wave or shock wave. The sound wave passes through the ocean water into the sediment and is partially reflected at each depth where the type of sediment changes or a volcanic rock layer begins (Fig. 3-9). Because ocean sediments are built up in layers, many echoes, corresponding to the top of each layer, are reflected from within most ocean sediments. Returning echoes are monitored with a string of hydrophones, devices that record sound waves, towed behind the research vessel. The sound waves received by each hydrophone have traveled different distances from the top of each sediment layer and are received at different times (Fig. 3-9a,b). Seismic profiles reveal the sediment’s structural features hidden below the seafloor, including faults, tilting of the layers, reef structures, and buried mountain tops. Modern seismic profiling systems use multiple sound sources and multiple strings of hydrophones towed behind the research vessel. Using powerful computers to analyze the resulting data, these systems can produce detailed three-dimensional images of the sediments or rock beneath the seafloor (Fig. 3-9c,d).

Figure 3-9. Two seismic profiling techniques are used to map subsurface sediment layers and structures. (a) Seismic reflection sends sound pulses, which penetrate the seafloor and are reflected off sedimentary layers. (b) A typical reflection seismic profile shows rugged seafloor topography overlaid by sediment layers. (c) Seismic refraction sends sound pulses, which are refracted (bent) at interfaces between layers of sediment and then travel back out to be received by a second ship. At one time, the sound source was provided by dynamite sticks thrown overboard to explode just under the water surface. However, the sound source typically used now is an air gun that periodically fires a burst of compressed air into the water at a depth of about 15 to 35 m. The sound waves are generated by the collapse of the air bubbles produced by each burst. (d) A 3-dimensional seismic refraction image of an ancient reef structure buried beneath the sediments off Nova Scotia, Canada. The light colored 2-dimensional sections across the center of the image and along the right side show how much more information the 3-dimensional image can convey.

Because sound travels at different speeds in different types of sediment and rock, seismic profiles can help determine the nature of each layer of sediment. Distinct sediment layers in many parts of the ocean can be traced for hundreds of kilometers in all directions. Hence, the layers found in seismic maps can often be correlated to the layers of sediment and rock retrieved by drilling or coring.

Sediment and rock on or below the seafloor can also be studied by precise measurement of changes in the strength of the gravitational-field or magnetic-field strength. Tiny changes in magnetic-field strength are detected by instruments towed behind a research vessel as it passes over seafloor sediment and rocks with variable magnetization. Gravitational-field strength also changes slightly at different locations because the Earth is not perfectly round, and because denser, heavier sediments and rocks exert a slightly greater gravitational pull than less dense or lighter sediments at the same depth. Therefore, extremely small changes in gravitational-field strength detected by instruments carried on research ships provide information about the sediment and rock below, especially the presence of mountains of volcanic rock overlain by less dense sediment.

Dredges

Although most ocean floor is covered by thick sediment, rocky outcrops also are present. Some of these rocky areas of the seafloor are partially covered with manganese nodules and phosphorite nodules (Chap. 8). Because corers and drills cannot sample such surface rocks, dredges are commonly used. A dredge consists of a metal chain or nylon mesh net with one end held open by a strong, rigid, metal frame (Fig. 3-10).

Figure 3-10. (a) Rock dredges are used to obtain rock samples, including nodules, from the seafloor with little or no sediment cover. (b) The dredge is lowered on a cable and towed across the seafloor. Sediment passes into and through the net, while nodules and loose rocks are retained. The metal collar breaks off rocks from a rocky seafloor.

Dredges are often massive because the rocks in many areas must be broken off solid lava flows. When a dredge snags on such rocks, considerable force is needed to break the rock and release the dredge. Therefore, the cable used for lowering and towing dredges must be extremely strong. Steel cables 1 cm or more in diameter are sometimes required. In very deep water, 10,000 m or more of cable weighing several tonnes is required. Because the drum on which the heavy dredge cable is stored and the winch needed to drive the drum are huge, only a few of the largest research vessels can deploy the largest dredges. Submersibles, ROVs, or AUVs discussed later in the chapter now perform some of this type of sampling.