Marine Biology (2+1)

Sea bottom Sampling devices

Sea bottom sampling devices

Several types of technology are used to collect marine sediments from research ships. These devices include surface samplers and sediment corers. Surface samplers collect only the uppermost layers of the ocean floor. These devices include dredges and benthic grabs. Dredges are heavy nets attached to metal frames that are dragged along the bottom behind a ship. The nets themselves can be made of chain. Dredges are useful when collecting samples from hard surfaces such as the rocky bottoms of coastal habitats.

Benthic surface grab samplers look like giant metal jaws. They dig into the bottom and take a bite of the sediment. These samplers are good for collecting softer, sandy or silty sediments that do not contain rocks. A box corer is a cross between a surface sampler and a sediment corer. It is a special device that is used to collect an undisturbed sample of the top surface layers and the sediment underneath.

This corer collects a 1-2 foot thick “box of mud”. A box core usually contains some overlying water, a surface “floc” layer where newly deposited particles are suspended, and frequently the core collects some benthic animals such as worms and starfish. Box corers are used to collect mud and silt but not sand.

The most common types of sediment corer used by marine geologists to collect mud and silt are those that collect a tube of sediment. The tube of sediment collected can range in length from several feet to hundreds of feet. A gravity corer consists of a metal tube called a barrel, a plastic

liner, and a heavy weight. The plastic liner is inserted into the metal tube before it is lowered over the side of the ship. This plastic liner will contain the sediment core when it is retrieved from the ocean floor. The corer is lowered to the bottom where the weight on the top drives the barrel and liner into the sediment. Gravity corers generally sample sediment layers up to 6 feet long (or 2 meters). After collection, the sediment core within the hard plastic liner is slid out of the core barrel, caps are placed on both ends, and the core is stored in a large walk-in refrigerator. Scientists will split the core in to 2 halves  length-wise  “working half”[Nazeer1] . is used for various analyses.  The other core half is saved as the “archive half”.

Piston corers look like gravity corers, but they are much longer, much heavier and collect a longer core sediment. A cylindrical piston system at the top of the coring unit, which can slide down the core barrel with the help of weights, acts to drive the corer deep into the sediment. This gives scientists a very compact, long sediment core with very little water. Due to the force with which the piston core is driven into the mud, the very top is often lost and not sampled. However, this corer is able to penetrate much deeper into the seafloor, thus collecting much older sediments than what a gravity corer or a box corer can obtain. This is because the deeper the sediment is below the surface of the seafloor,the older it is.

Several other types of coring devices are also useful to marine scientists. The multicorer punches 6 or more, 1-2’ long tubes into the bottom, all at once. It also relies on gravity and weights and is most useful in sticky, muddy sediments. On a really big scale, the international Deep-Sea Drilling Project (DSDP) uses drilling technology from the oil and gas industry to obtain cores of 10’s –100’s of meters long. These long cores give scientists an excellent overview of the geologic and oceanographic history over thousands of years. The cores are obtained from a specialized ship, designed to operate like an oil drilling platform used to collect oil from beneath the seafloor. Obtaining cores that are hundreds of meters long from a ship takes many hours. This process can be very challenging and occasionally dangerous! This is especially true when the seas are rough and/or there is a lot of ice in the areas such as in the Antarctic. In general, the success or failure of all types of sediment coring done from oceanographic research ships is very much dependent upon the sea state.

Today various types of underwater vehicles are also being used to collect sediment samples. Submersibles or remotely operated vehicles (ROVs) collect sediments using small push corers less than one foot long. These plastic tubes are pushed into the surface sediment by the submersible vehicle’s/ROV’s mechanical or hydraulic arm. The recovered sample is placed in a sample bucket or cage on the front of the underwater vehicle and is transported back to the ship. The advantage of this type of sediment sampling is that the scientist, either inside the submersible or controlling the ROV with the use of cameras from the ship, can visually target the sampling site. All other shipboard coring methods described above are done “blindly”—the corer is lowered over the side of the ship to the seafloor without the scientist being able to see exactly what is being sampled.