33 



an accurate picture of oxygen variation in the very shallow waters, but it 

 was possible to get some idea of average oxygen concentration at all depths 

 from over 3,000 meters in the entrance of the Gulf to about 40 meters along 

 the coast (fig. 9). 



These data were first isoplethed at .5 ml/L. intervals, and then smoothed 

 out to give the picture of oxygen distribution as shown in fig. 9. As can 

 be seen in the northern Gulf, there is normally an orderly progression from 

 high oxygen to low oxygen from shallow to deep water, but the presence 

 of the "oxygen-minimum zone" from Tiburon Island to the south com- 

 plicates the picture considerably. Values drop suddenly at about 100 to 

 200 meters to below .5 ml/L., continuing with low values to depths of 

 about 1200 meters, where they rise to 1 ml/L. in the southern basins and 

 even higher at the entrance to the Gulf. One exception to this pattern can 

 be found in the Guaymas Basin, where there is less than .5 ml/L. at the 

 bottom. This basin also contains the largest concentration of diatomites. 

 These diatomites are finely layered, and show no signs of disturbance by 

 burrowing organisms. The lack of animals necessary to disturb layered 

 sediments can probably be attributed to the absence of enough oxygen 

 to sustain macro-invertebrate life. Similar adverse conditions have been 

 described by Emery and Hulsemann (1962) in the Santa Barbara Basin 

 in the continental borderland off Southern California. This basin is some- 

 what shallower (550 meters), but oxygen values are quite similar and even 

 lower than the Guaymas Basin, ranging from .5 ml/L. at sill depth of 475 

 meters to .1 ml/L. at the bottom. As in the Guaymas Basin, sediments are 

 neatly layered, showing graded deposits resulting from turbidity current 

 deposition, interspersed with alternating diatomite and terrigenous deposits. 

 Upwelling and related high plankton productivity although not on a seasonal 

 basis brings about a depletion of oxygen at the bottom, as in the Guaymas 

 Basin. Thus, during times of very low oxygen, fine layers are preserved 

 owing to lack of benthic organisms. During non-upwelling portions of the 

 cycle, there is an increase in oxygen permitting the existence of burrowing 

 animals which destroy some of the laminae. The primary difference be- 

 tween the Santa Barbara Basin and Guaymas Basin is that oxygen 

 depletion seems to be more or less permanent in the Guaymas Basin, since 

 no disturbance laminae occurred anywhere in the Guaymas cores, and no 

 signs of life have been found in dredge samples. 



The areas with lowest oxygen values on the upper portions of the slope 

 of the Gulf of California can be correlated quite closely with the areas of 

 intense upwelling and associated plankton or diatom blooms, as shown 

 in fig. 10 (taken from Roden and Groves, 1959, and Byrne and Emery, 



3 Vidensk. Medd. fra Dansk naturh. Foren. Bd. 126. 



