CHAPTER L 



BOTTOM SEDIMENT SAMPLING 



L-1 General. — Marine sedimentation em- 

 braces that phase of oceanography which is 

 related to the deposition, composition, classifica- 

 tion, and stiiicture of organic and inorganic 

 material of the ocean floor. Various sampling de- 

 vices are utilized to obtain bottom sediments 

 from a particular locality under investigation. 

 Once obtained, samples are packed and shipped 

 to a sedimentation laboratory to be analyzed 

 and classified. 



Analysis of marine sediments generally in- 

 cludes the determination of size, shape, and per- 

 centage of component particles; identification of 

 minerals and ratio of light to heavy minerals; 

 wet density; pH; and calcium carbonate con- 

 tent. Biological and ecological studies empha- 

 size the animal population as well as the 

 environmental factors determined by tempera- 

 ture, depth, type of sediment, and geographic 

 location. 



Classification of bottom sediments is based on 

 a combination of grain size and genesis (ori- 

 gin). Hence, one may have samples composed 

 of terrigenous material, subaerial or submarine 

 volcanic material, organic matter, inorganic 

 material, and extraterrestrial matter. Size of 

 the component materials may range in extremes 

 and is used as a further, more detailed classifi- 

 cation criteria. Bottom samples composed of 

 volcanic materials for example, may range in 

 size from very fine ash to pebbles and cobbles. 

 Very often bottom samples contain an aggre- 

 gation of sizes so that a combination of vol- 

 canic ash and other material of pebble size is 

 possible. 



Bottom sediment charts are prepared from 

 thousands of reported classifications and col- 

 lected samples. These charts illustrate the na- 

 ture of the sea bottom in coastal and oceanic 



L-2 Collecting Samples. — Collecting marine 

 sediments involves the use of a variety of sam- 

 plers which fall into three basic categories: 

 Corers, snappers or grabs, and dredges. Selec- 

 tion and use of the proper device will depend 

 on the nature of the investigation, the character 

 of the bottom, the depth of water, and the ship- 

 board equipment available for lowering and re- 

 trieving the samplers. For example, if the in- 

 vestigation has to do with the strength of the 



sediment or its ability to support equipment, the 

 sample should be obtained with one of the 

 larger corers so that engineering properties as 

 well as size and composition analyses can be 

 made. On the other hand if previous reconnais- 

 sance indicated the character of the bottom to 

 be hard and rocky, perhaps a dredge or grab 

 sample will verify this condition. Where the 

 depth of water is great and the sediments are 

 unconsolidated, excessive washout may elimi- 

 nate the use of certain devices. 



L-3 General Procedures for Coring Opera- 

 tions. — Coring operations aboard an oceano- 

 graphic survey ship are guided by procedures 

 established to facilitate the collection of useful 

 samples, insure a maximum degree of efficiency, 

 and provide for safety of personnel in handling 

 of coring equipment. 



The typical coring device consists of inter- 

 changeable core tubes and an upper assembly. 

 The upper assembly provides support for the 

 drive weights and the core tubes. These corers 

 essentially are driven into the ocean floor by 

 gravity, and the bottom sample is retained in 

 the core tube. The time involved in a coring 

 operation is dependent on water depth and the 

 speed at which the wire is payed out and re- 

 trieved. The length of core collected will be 

 governed by the penetrability of the bottom, the 

 length of the corer, tlie amount of weight on the 

 device, and the design of the corer. In areas of 

 predominantly rocky or coral bottoms, it may be 

 impossible to obtain a core. 



L4 Gravity- and Piston-Type Corers.— 



Aboard the survey ships of the U.S. Naval 

 Oceanographic Office, two types of coring 

 devices are used : Gravity-type and piston-type 

 corers. Both types of corers achieve their pene- 

 tration of the ocean floor by gravity, i.e., when 

 the release mechanism is tripped, the specific 

 gravity of the device is great enough to cause 

 the corer to free fall rapidly through the water 

 and strike the bottom with enough force to 

 penetrate the ocean floor (fig. L-1). Interest 

 in obtaining undisturbed core samples has 

 resulted in the development of several piston- 

 type bottom coring samplers. These piston-type 

 corers are designed to offset the downward 

 force of the coring device on the sediment. The 



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