20 4 DISCOVERY REPORTS 



BOTTOM DEPOSITS 



One of the most interesting features of the South-west African region is the presence, on the sea-floor 

 of the continental shelf, of an extensive area of diatomaceous mud. In contrast to the great deposits 

 of diatom ooze which occur in the deep ocean, this zone is confined to shallow waters, and by its 

 very proximity to the sea-surface has made its periodically catastrophic effects all the more evident. 



The extent of the diatom mud was outlined by Marchand (1928). He described it as extending 

 principally from 21 ° 30' S. to 24 30' S. (a distance of about 200 miles) and running seawards for 

 some 25-30 miles from the coast. The bottom samples from this area in his words 'have a green 

 colour and are of a muddy or clayey consistency. The stench emanating from them is unbearable and 

 somewhat similar to hydrogen sulphide or the odour characteristic of putrefaction.' 



Close to the coast, however, this mud was not found, and Marchand says that it gives place to a belt 

 of grey sand on which the marine fauna and flora were abundant. It appears, therefore, that the evil 

 smelling mud was confined to an area of the sea-bed between depths of about 50 and 1 50 m., extending 

 along the coast for some 200 miles. 



The absence of marine life in this diatom mud, and the uselessness of the ground for trawling, led 

 to the name by which it is now known, the ' azoic zone ' (strictly speaking the name ' anaerobic zone ' 

 would be more precise, for the mud does in fact support bacteria and is, therefore, not ' azoic '). 



Copenhagen (1934) examined the deposits in the azoic zone closely and concluded that the evil smell 

 was in fact hydrogen sulphide, and that it originated from the activities of sulphate-reducing bacteria 

 (Butlin, 1949) which were shown to be present in the sediment. Analyses of the sediment revealed a 

 high content of organic matter and, on drying, a whitish-yellow deposit, evidently of sulphur, became 

 apparent on the surface of the mud. 



Microscopical examination of the sediment has shown it to be composed mainly of diatom frustules, 

 although towards the seaward limit of the zone foraminiferal remains become increasingly abundant 

 (Marchand, 1928). 



The importance of the azoic zone in the circulation of the waters of the Benguela current became 

 evident when the results of survey I were examined. It was decided, therefore, on survey II to include 

 a series of bottom sampling stations in an attempt to delineate more clearly the total extent of the 

 zone. Samples were taken to the south and to the north of the limits noted by Marchand (1928), and 

 these have shown that the azoic mud extends, at least patchily, for some 400 miles along the coast, 

 from Cunene river mouth (17 30' S.) in the north, to about 25 S. Seawards of the deposit is a fairly 

 rapid change to the stone-grey globigerina ooze which characterizes the sediments of the continental 

 slope and floor of the deep ocean to the west. 



It is particularly interesting, however, that the azoic mud was not found on the Orange river line, 

 nor in the trawl at station WS 990 on survey I, and this suggests that the southern limit must indeed 

 be somewhere in the region of 25 ° S. The sediments on the shelf south of the azoic zone do not 

 differ markedly in appearance from the azoic mud, but they have none of the offensive smell of the 

 latter. 



Reference to Figs. 38 and 41 shows that over this area the oxygen content of the water overlying the 

 sea-bed, exemplified by the Orange river line, was by no means so depleted as that found, for example, 

 off Walvis Bay over the azoic zone (cf. Figs. 40 and 43). This, in turn, relates to the higher content of 

 dissolved oxygen in the water which was being upwelled on the Orange river line (as shown by the 

 oxygen content at 200-300 m. off the edge of the continental shelf) as compared with that upwelling 

 further to the north. 



The evidence, therefore, points strongly to the probability that this diatomaceous sediment to the 



