and no sand grains are present,, visible animal markings and 

 impressions indicate a lack of current activity. 



Currents are normally very small in deeper clay 

 areas, but occasional banking of fine-grained sediments 

 against nodules and rock outcrops indicates the presence 

 of low-order water motion. Sometimes the bending of 

 bottom-attached organisms and deformed bottom crawlers 

 indicate weak sea floor currents. 



Current meters attached to camera frames are now 

 being used to measure water velocities at the sea floor in 

 various environments. 



Microrelief Resulting from Chemical Precipitation 



Of all the microstructures that have been discovered 

 on the sea floor, the manganese nodule, slab, or encrusta- 

 tion is perhaps the most interesting. It may have influence 

 on sound reflectivity when concentrated, and it will probably 

 have economic value to man in the future. 



The manganese nodule, like phosphorite and ferric 

 nodules, is a product of chemical precipitation that takes 

 place at the sea floor (figs. 17 and 18). The manganese 

 nodule usually contains small percentages of iron, cobalt, 

 copper, and nickel oxides. Goldberg and Arrhenius 39 be- 

 lieve that organisms concentrate dissolved chemicals in the 

 sea water column, after which chemical changes and attach- 

 ment to various nuclei on the bottom occur. These authors 

 have also suggested that there is a direct correlation between 

 the iron/ manganese ratio in nodules and the rate of sedi- 

 mentation. Graham 30 believes that organic reactions in 

 seawater form manganese nodules. Although the mode of 

 formation of the nodules is not yet clear to the oceanographer, 

 the deposits are substantial and form recognizable relief. 



The heights and lengths of the nodules range from 

 less than 1 mm to 50 cm or greater, but the nodules for a 

 particular area usually fall within a narrow size range. 

 The concentration (number of nodules per square meter of 

 bottom) is difficult to determine. Sonar and TV" scanning 



40 



