BOTTOM DEPOSITS 205 



south of the azoic zone is maintained in an aerobic state by the circulation over it of upwelled water 

 which is not heavily depleted of oxygen. 



Another factor, however, which must be considered is that the northerly winds and calms experienced 

 at Walvis Bay (p. 199) do not appear to prevail on the stretch of coast south of 25 S., and their absence 

 may ensure a free circulation of the current over the sediment ; but it is felt that these meteorological 

 conditions may concern more implicitly the intensity of the development of the anaerobic region 

 rather than determine the presence or absence of such a zone. 



The presence of anaerobic conditions is essential for the growth of sulphate-reducing bacteria, and 

 if the sediment is kept oxygenated they cannot survive. 



We have already seen (p. 198) how particular conditions within the current may magnify the effects 

 of hydrogen sulphide production, and indeed at times it is even evolved actively from the sea-surface. 

 There are many observations of bubbles of the gas rising to the sea-surface, but perhaps the following 

 newspaper account gives the most graphic description! (News Chronicle, 1938): 



SEA GAS ATTACKS TOWN 



Swakopmund, the small coastal town in strewn on beach, sharks come into surf gasp- 

 South-west Africa, is undergoing a gas attack ing on evening tide. 



set up by continuous submarine disturbances. Cause. A geologist, discounting volcanic 

 Heavy sulphurous fumes, especially towards action, says sulphuretted hydrogen, pro- 

 evening, are penetrating as far as 40 miles duced by bacteria on the sea floor from cal- 

 inland. cium sulphate or gypsum, accumulates until 



Effect. Atmosphere like a London fog, it raises islands of mud, which eventually 



metalwork turned black, public clocks burst, 

 blotted out by deposit, thousands of fish 



In 1 95 1 a particularly active disturbance took place, and on 16 February the Bulawayo Chronicle 

 (Anon., 1951a) reported: 



SEA ERUPTIONS OFF SOUTH-WEST AFRICA 



Swakop. S.W.A. Thurs. species, such as sea snakes and eels, which 



The stench of sulphurous sea eruptions are found only occasionally when the sea 



pervades the air day and night. Buildings bottom of their habit undergoes violent 



which were white yesterday, are discoloured disturbance. 



and black today. The sea appears to be boiling with bubbles 



Dead fish are being washed up along the rising to the surface, but the temperature of 



beaches for over 100 miles to the north from the water is only 6o°-70° F. 



Swakopmund. They include many strange 



These eruptions continued through March and three mud islands were thrust up in Walvis Bay 

 (Anon., 195 1 b). The largest, about 100 yards long, disappeared overnight. 



The islands were similar to one which was thrust up in 1900. Waldron (1910) reported on the latter, 

 from which samples were taken by an officer of a regiment at Walvis Bay, who swam out to the island. 

 The samples were typical of the mud lying on the sea-bed in the bay, and the temperature of the 

 water remained low in spite of the fact that 'steam' was observed to issue from the northern side of 

 the island. The appearance of the island was accompanied by a very strong smell of hydrogen sulphide. 

 As yet no satisfactory explanation of the formation of these islands has been reached. 



The importance of the sediment of the azoic zone in yet another way has been emphasized by 

 Brongersma-Sanders (1948) who has shown the significance of this anaerobic deposition of organic 

 matter to oil geology. It is noteworthy how closely the conditions of deposition within this upwelling 

 current parallel those found associated with fossil deposits. 



Study of this diatomaceous mud— first distinguished by Neaverson (1934) from the nearly colour- 

 less diatomaceous ooze of some deep-sea deposits — may also yield interesting evidence on the silicate 



