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Sediments of the deep basins of the central and southern Gulf of California contain high concentrations of diatom and radiolarian tests. Determinations of the amounts of opal in the sediments, by X-ray diffraction, show that the diatoms contribute the greater proportion of biogenous SiO2 to the sediments. An opal-rich area is confined to the central Gulf, which is also the site of most intense phytoplankton production. Over an area of approximately 2,500 km.2, sediments containing 50 per cent by weight opal are accumulating at a rate of ~3 mm./yr. Within this region, the ratio of the rate of accumulation of opal to that of terrigenous material is highest for the whole Gulf.
The Gulf is in open communication with the Pacific Ocean, and the annual exchange of water through the mouth is estimated to be ~5×1016 liters. With northerly winds, the surface waters, depleted of plant nutrients, are driven out of the Gulf, to be replaced by upwelled nutrient-rich waters flowing into the Gulf at depth from the Pacific. In this way, sufficient silica is supplied to the euphotic zone, where it is utilized by diatoms, to account for all the silica known to be accumulating on the floor of the Gulf. River supplies are 100 times less than the ocean supply. In view of numerous observations of a so-called genetic association between diatomaceous sediments and volcanism, the mechanism operating in the Gulf of California should be considered as a means of conc ntrating dissolved silica, at concentrations of 1-2 ppm, as siliceous sediments. Sufficient silica is present in sea water, in the form specifically utilized by diatoms, to form deposits of purity comparable with the Monterey diatomites of California.
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