28. ORGANIC TRANSPORTATION OF MARINE SEDIMENTS 



K. O. Emery 



1. Introduction 



The chief agents of erosion of the ocean floor are inorganic in nature; however, 

 organic agents locally may be more important. Organisms make use of both 

 mechanical and chemical methods of erosion. Simple mechanical erosion is 

 illustrated by the biting off of coral tips by fishes in search of the polyps within, 

 and by the breaking away of projecting rocks by attached kelp lifted by storm 

 waves. Erosion of rock by pholads, snails, chitons, limpets, echinoids and 

 worms (Barrows, 1917; Emery, 1960, pp. 15-19) is also mostly mechanical in 

 nature, but is aided to an unknown extent by biochemical solution of the rocks. 

 Sponges and blue-green algae (Ranson, 1955; Revelle and Fairbridge, 1957, pp. 

 279, 280) appear to do their work mostly by biochemical methods. Biochemical 

 activity alone, or almost alone, is responsible for the development and growth 

 of solution basins and the leveling of reef flats (Revelle and Emery, 1957). 



Organisms transport and deposit some sediments which are produced by 

 inorganic agents as well as those which they themselves eroded. The organic 

 transporting agents play roles which make difficult an assignment of order of 

 importance. For example, worms probably transport a greater quantity of 

 sediment than do all other organisms combined, but the movement is only a 

 few centimeters; in contrast, driftwood and birds carry small quantities for 

 thousands of kilometers. Each of the major kinds of transporting organisms 

 will be considered in turn, but in a more or less arbitrary order. 



Many oceanographers appear to know of several instances of organic trans- 

 portation of stones, but few realize the extent of this transport or the number of 

 organic agents involved; accordingly, a relatively large bibliography is included 

 for its possible value in other studies. 



2. Kelp 



The larger seaweeds, particidarly the genera of larger brown algae, are known 

 as kelps. Largest of these is Macrocystis, which reaches lengths of 200 m, and 

 Pelagophycus and Nereocystis — about 30 m. All of these plants live in waters 

 mostly between 5 and 20 m deep, exceptionally 30 m. Several other genera 

 {Egregia and Pterygophora) occur in shallower water, up to the level of low tide. 

 Gas-filled floats keep most of the plant near the water surface where the blades 

 spread out in a broad canopy to catch the sunlight. At the base is a holdfast, 

 or rootlike mass, composed of branching structures attached to the bottom. 

 Usually attachment is to bedrock or to boulders and cobbles, but many hold- 

 fasts also enclose much shelly fine gravel and sand and some are imbedded only 

 in sand (Thompson, 1959). The writer has also seen two holdfasts of Macrocystis 

 attached to the tops of abalone {Haliotis) shells. 



Large waves from storms often exert a greater lifting force on the plant 

 and its floats than the attachment can stand. Unless the stipe, or stem, parts 



[MS received March, 1960] 776 



