42 A TEXTBOOK OF OCEANOGRAPHY 



4 per cent. Calcium carbonate disappears with increase of 

 depth. 



Down to 1,000 fathoms nearly every shell of pelagic 

 (surface) organisms is represented in the deposit, even the 

 smallest and most delicate. At 1,500 fathoms the thinnest and 

 smallest shells disappear, and pteropod ooze passes gradually 

 into globigerina ooze. 



At 2,000 fathoms the pteropods have disappeared entirely, 

 and some of the more delicate foraminifera as well. At 2,500 

 fathoms the larger and thicker foraminifera still remain, and 

 the deposit becomes a red clay with some carbonate of lime. 

 At 4,000 fathoms hardly a trace of these shells can be found, 

 and chemical analysis shows barely i per cent, calcium 

 carbonate. 



And this although the living organisms at the surface are 

 as abundant over the red clay areas as over the pteropod ooze 

 areas. It is at about 2,500 fathoms that the percentage of 

 calcium carbonate in the deposits falls off very rapidly. 

 According to Murray, it would take from three to six days 

 for shells to reach a depth of 2,500 fathoms. Calcium under- 

 goes extensive circulation between the dissolved and undis- 

 solved states in the ocean. When calcareous fragments fall 

 on a clay or mud bottom they fall into water which can take 

 up lime, and are dissolved. When they fall in calcareous 

 deposits, such as pteropod ooze, they fall into water layers 

 which can dissolve no more lime. In areas over globigerina 

 and pteropod oozes lime is being withdrawn from the ocean. 

 Over red clay areas lime is returned to the ocean. Probably a 

 balance is struck, though, on the whole, lime at the present 

 time appears to be accumulating towards the Equator. 



Red Clay is characteristic of the greatest depths on the ocean 

 floor. It is the most widely distributed of deep-sea deposits. 

 First discovered by the Challenger in depths exceeding 2,400 

 fathoms between Tenerife and the West Indies, it was first 

 thought to be the ultimate sediment produced by disintegration 

 of the land Wyville-Thomson, the leader of the Challenger 



