January 12, 1922] 



NATURE. 



45 



Oceanography of the Gibraltar Region. 



By Dr. Johs. Schmidt. 



THE first month spent at sea by the Dana Expedi- 

 tion was occupied witli investigations in the 

 mndary area between the Atlantic and the Medi- 

 iranean — i.e. in the Bay of Cadiz, the Straits of 

 ( libraltar, and the Western Mediterranean as far as 

 Algiers. We had worked there l)efore, in 1908-10, 



S/fS 



ree/ Step 



I. — Showing isohalines (figures in italics) and isotherms. Hydrographical section through 

 the Straits of Gibraltar from the investigations of the Thor, February, 1909. (Schmidt, 

 Nielsen, and Jacobsen, 1910.) 



with the Thor. We were, therefore, not un- 

 acquainted with local conditions, and it was very 

 interesting to compare the new investigations with 

 the old. The expedition was particularly fortunate 

 in being able, during this month, to enjoy the co- 

 operation of Dr. J. N. Nielsen, who, from his par- 

 ticipation in the Thor expeditions, is perhaps more 

 familiar with the hydrography of 

 these waters than anyone else. The 

 remaining scientific staff of the ex- 

 pedition consisted of Messrs. P. 

 jespersen and A. V. Taning, both 

 ichthyologists and trained in the 

 work of general marine biology ; 

 K . Stephensen, as expert in crusta- 

 ' cans ; while the physical and 

 ' hemical investigations were car- 

 ried out by Messrs. J. Olsen and 

 ' . C. Andersen, the last-named 



ing physician to the expedition. 



Previous investigations — British, 

 Danish, and Norwegian — have 

 gi\en us the main features (but no 

 more) in the transfusion of water 

 which takes place between the 

 Mediterranean and the Atlantic. 

 The most striking difference 

 between Mediterranean and Atlantic water is in the 

 salinity. Owing to the great evaporation, the water 

 ■n the Mediterranean is of higher salinity than the 



ilantic water — viz. more than 38 per mille (that is, 

 ^ grams of salt in 1000 grams of sea-water) as 



iinst about ,;6-36-5. The less saline Atlantic water 



US through the Straits of Gibraltar into the Medi- 

 NO. 2724, VOL. 109] 



terranean as a surface current. Deeper down, the 

 Straits of Gibraltar are filled with salter water, which, 

 coming from the Mediterranean, moves westward 

 over the comparatively shallow threshold, in places 

 only 400 metres down, formed here by the sea floor, 

 which falls away steeply both to the east and west. 

 On reaching the western edge of 

 this ledge, the Mediterranean water 

 pours down, like a veritable sub- 

 marine waterfall, towards the 

 depths of the Atlantic Ocean 

 (Fig. i). It does not, however, 

 reach the bottom ; being warmer, 

 and therefore lighter, than the 

 bottom water of the Atlantic, it 

 spreads out as an intermediate 

 layer in the Atlantic, which is re- 

 cognisable in that it is of higher 

 salinity than the water-layers above 

 and below. J. N. Nielsen and F. 

 Nansen have previously demon- 

 strated the existence of this inter- 

 mediate layer of Mediterranean 

 water — or, more correctly, mixed 

 water — right up to the west of the 

 British Isles. At the stations of 

 the Dana Expedition between 

 Madeira and the Cape Verde Islands (Stations 1142, 

 1 152, 1 156, 1 157, 1 159) we found it at depths of 

 about 1000-1500 metres, with a salinity naturally 

 decreasing towards the south, but varying from 

 about 35-7 per mille at Madeira, to about 35-03 

 near the Cape Verde Islands. 



Up to now we have referred only to the outflow 



uoo^ 



Fig. 2.— Hydroeraphical section through the westernmost portion of the Mediterranean as shown 

 by the Dami's stations 1119, 1120, and lui, about October 1, 1921. 'I'he isohalines for 

 36"5. 37'o. 37'5. S^'o, and 38*3 per mille salinity are shown. The depths are given in 

 metres. The section shows that the inflowing Atlantic water (salinity less than 36'5 per 

 mille) follows the coast of Africa. 



of Mediterranean water into the Atlantic. Fig. 2 

 gives a picture of the inflow of Atlantic water into 

 the Mediterranean, based on the investigations of 

 the Dana Expedition in the waters between Oran, 

 Algeria, and the south coast of Spain. The figure 

 represents a vertical section of the upper 400 metres 

 of sea through our three stations 11 19, 1120, and 



