The Oceans Their Physics, Chemistry, and General Biology "d3_8

Bottom-Sampling Devices

Devices used for collecting specimens of the sea-bottom sediments depend upon the character of the deposit, the depth of water, and the strength of the wire rope available. Certain apparatus that is suitable for work in soft, cohesive sediments cannot be used where the bottom material is coarse-grained or where it is rocky. Similarly, other types that can be operated in shallow depths are too heavy for general use in deep water. Methods have been devised for sampling the superficial layers of the sediments, but since the Meteor Expedition, 1925–1927, greater emphasis has been placed upon obtaining core samples. Instruments are now in use that will take cores several meters long, and much thought is being given to the development of instruments that will take even longer samples. Unlike the equipment used in most of the other fields of oceanography, the devices for taking samples of the marine sediments are not standardized. Every investigator uses his own type of bottom sampler, but all samplers are based on certain basic designs. Hough (1939) has listed and described the various types and gives an exhaustive bibliography.

Bottom samplers used for oceanographic work fall into three general categories; dredges (drag buckets), snappers, and coring tubes. Dredges patterned after the naturalist dredge (fig. 89), but constructed of stronger material and with a chain-mesh bag, are used for procuring samples of rock where the sea bottom is covered with rock fragments or where there are outcrops of solid rock. Smaller cylindrical dredges with solid bottoms

― 345 ―

are sometimes used for collecting nonconsolidated material in relatively shallow water. Snapper samplers of the clamshell type have been widely used for obtaining samples of the superficial layers of the sediments. The telegraph snapper (fig. 84) has been widely used, particularly in routine wire-sounding work by surveying vessels. The drawback to the use of this relatively simple device is that it takes so small a sample. The same principle has been employed in the Ross snapper, which will hold several hundred cubic centimeters of sediment. Other samplers of this general type have been devised for sampling the benthic fauna (fig. 89 and p. 375). The disadvantage of the clamshell type of sampler is that its contents are likely to be washed out while it is being hauled to the surface. This is particularly true when it is used in areas where the bottom is sandy or contains coarse fragments, since a fragment caught in the jaws may prevent them from closing completely.

Bottom sampler—snapper type.

[Full Size]

Coring devices (fig. 85) are essentially long tubes that are driven into the sediment, either by their own momentum or by the discharge of an explosive. The latter principle is used in the coring instrument developed by Piggot (1936). A momentum-type coring instrument weighing about 600 lb with the weights attached will take cores up to about 5 m in length in soft sediments at depths as great as about 2000 m (Emery and Dietz, 1941). The Piggot coring tube has been used to take cores about 3 m long at depths greater than 4000 m. In coarse-grained deposits the coring tubes are not capable of penetrating more than about 0.5 m.

The momentum-type core sampler is allowed to run out freely when it approaches the bottom. The depth of penetration is determined by the weight of the instrument, the character of the sediment, the diameter of the tube, and the type of cutting nose. The factor that determines the size and type of coring tube which can be used on any wire rope is not the weight of the instrument alone, because the greatest strain is developed when the coring tube is pulled out of the sediment, and this may be several times the weight of the instrument. The Emery-Dietz sampler is constructed of galvanized iron pipe of 2- or 2.5-inch diameter. It is connected through a reducing coupling to a smaller pipe on which the weights are mounted. The reducing coupling is perforated to permit the passage of water, and is sometimes fitted with a ball valve. The cutting nose is sharp, with a slightly smaller internal diameter than the tube itself so as to reduce the internal wall friction and to hold in the inner liner, which further reduces the wall friction and facilitates the removal of the core samples. Inner liners are sheets of metal or celluloid rolled up and inserted in the pipe, or they are glass or metal pipes that are cut open after the sample is taken. In some coring devices a “core catcher” is fitted to the nose of the coring tube to prevent the core from sliding out of the tube after the instrument is raised from the bottom. The depth of penetration of coring tubes is generally considerably greater than the length of the sample obtained. The amount of “compaction” varies between about 25 and 50 per cent, depending upon the characteristics of the coring tube and the type of sediment. The nature of the compaction and the effects that it may have upon the stratification of the sample have been investigated by Emery and Dietz (1941). Smaller instruments based on an original design by Ekman and developed by Trask (Hough, 1939), but of the same general type described above, can be used on fairly light gear and will take samples between 0.5 and 1.0 m in length.

― 346 ―

The Emery-Dietz core sampler.

[Full Size]

― 347 ―

The care and treatment of bottom samples depend to a large extent upon the nature of the examinations that are to be made later (chapter XX). For most purposes it is sufficient to place the specimens in mason jars fitted with rubber washers. No preservative or additional water is necessary. Labels should be placed on the outside of the bottle, since mechanical abrasion and the activities of microorganisms may destroy or render illegible any paper in contact with the sediment sample. Core samples may be cut in sections and carefully placed in bottles, or the entire specimen may be kept in the inner liner.