Factors affecting the distribution of silicate in the North Atlantic Ocean and the formation of North Atlantic deep water

Abstract

(Text-figs. 1-2)In the deep water of the eastern North Atlantic below 2000 m. the variations with depth of salinity, temperature, density, oxygen, phosphorus compounds and nitrate are quite small. By contrast the silicate content is doubled in a descent from 2000 to 4°00 m.The distinctive behaviour of silicate is revealed by diagrams ( Fig. 1) relating it to salinity, temperature, density and total phosphorus at station 2659 worked by R.R.S. Discovery II on 12 May 1950 (Armstrong, 1951; Cooper 1952, Table IV). The temperature-salinity diagram (Cooper, 1952, fig. 15, to 15°°m. only) suggests that between 1200 and 2000 m. we have to deal with simple mixing of the mean Gulf of Gibraltar and North Atlantic Deep waters. If silicate concentration were subject only to mixing processes the curves in Fig. 1 between these depths would be straight lines. They are not-consequently it would seem that solution of either particulate silica or of aluminosilicates may be occurring. As yet, clear interpretation is not possible. At least five hypotheses may be erected to explain, in whole or in part, the observed distribution: (i) solution of bottom deposits; (ii) solution of ' clay' and of silica in suspension; (ill) concentration by vertical partition; (iv) tundra drainage; (v) sinking of surface water. These are examined in turn. ' SOLUTIONOF BOTTOMDEPOSITSDiscovery station 2659 was near the continental slope, solution from which could have occurred. The results of Wattenberg (1937) and Koczy (1950), however, suggest that the converse process occurs, adsorption or chemical combination of silicate from the water into the material of the bottom or into hexactinellid sponges. SOLUTION OF 'CLAY' AND OF SILICA IN SUSPENSIONThe distribution may have come about by solutionof mineralsof terrigeneous origin. If this were so the content of aluminium should increase with depth in much the same way as does silicate, and a diagram relating silica in solution to aluminium in solution should approach a straight line.

Radiolaria : flux, ecology, and taxonomy in the Pacific and Atlantic

“…Since seawater is undersaturated with respect to biogenic silca (e.g., Cooper, 1952;Hurd, 1972), dissolution of biogenic silica occurs throughout the course of sedimentation. The extent of dissolution is dependent upon the taxa, since morphology, chemical composition and residence time in the water column can vary a grea.t deaL.…”

The physical and chemical oceanography of the waters bathing the continental slope of the Celtic Sea

“…Between Armauer Hansen stations 7 and 15 (Gaarder, 1927) the temperature of this water ranged from 8-37 to 10-44°C. and the salinity from 35'52 to 35-75%0' whereas the oxygen content lay always between 65 and 68-5 % saturated-This property will sharply distinguish Gulf of Gibraltar water from cascaded or 'capsized' water (Cooper, 1952a) of similar temperature and salinity, but not from North Atlantic Central water which at similar depths has similar oxygen percentage saturation-…”

“…Off the Iberian Peninsula, Thomsen's (193 I) results showed that the nitrate-phosphate ratio in the newly formed Gulf of Gibraltar water, after correction for phosphate salt error, may be as high as 40: I. By the time it has flowed to the border of the Celtic Sea, this water may be still poorer than North Atlantic Central water at the same depth but, from the point of view of upwelling, it must be regarded as a relatively rich water, in which the anomalous nitratephosphate ratio has almost disappeared (Cooper, 1937(Cooper, , 1938a.…”

“…This deep water has been formed, according to Nansen (1912), chiefly at the surface in a limited area south-west of Greenland and to some extent by the outflow from the Norwegian Sea through the bottom of the Faeroe-ShetlandChannel (but see Cooper, 1952b). According to Helland-Hansen (1930, p. 75) a certain amount of Gulf of Gibraltar water appears to mix downwards to 5000 m.…”

On the distribution of silicic acid as a frontal zone tracer in the Indian sector of the Southern Ocean

“…nitrate and phosphate (Le Jehan and Tréguer, 1983;Le Corre and Minas, 1983). For these reasons, silicate would be an appropriate tracer of the frontal zones in the Indian sector of the Southern Ocean, as has been highlighted in the Atlantic Ocean by Cooper (1952), and later by Metcalf (1969).…”