Abstract

The contribution of biogenous silica to the sediments of the Gulf of California, and the factors controlling the formation of the deep-water sediment facies, have been determined by quantitative mineralogical analysis and by calculations of the rates of sediment accumulation. The amounts of biogenous silica (opal), quartz, and feldspar were determined by X-ray diffraction; calcium carbonate and organic carbon contents were determined by gasometric techniques; and rates of deposition were obtained by radiocarbon dating of sediment core sections.

Sediments containing more than 10 per cent by weight opal are confined to the central part of the Gulf; in the Guaymas Basin values exceed 50 per cent by weight. Diatoms constitute the major source of the opal in the sediments. Radiolarians contribute less than 10per cent by weight opal, and sponge spicules are an insignificant source. The distribution of the major sediment components reveals a deep-water sediment facies pattern dominated by biogenous supply (mainly opal) from the plankton and terrigenous supply from the rivers draining the Mexican mainland. This pattern is produced by a maximal rate of accumulation of opal, relative to terrigenous material, in the central part of the Gulf, particularly in the Guaymas Basin.

Phytoplankton production is greatest in the central Gulf, the area of richest diatomaceous sediments. The diatom frustules settle to the Gulf floor rather rapidly and are not dispersed uniformly over the entire Gulf as their individual settling velocities would predict.

The accumulation of biogenous silica in the sediments of the central Gulf proceeds at a rate of approximately 1013 g per annum. The rivers draining into the Gulf supply approximately 1011 g of dissolved silica per annum. The water exchange between the Gulf and the Pacific Ocean, as a consequence of the seasonal wind regimes, supplies approximately 1014 g of dissolved silica per annum to the Gulf of California. Sufficient silica is available in normal sea water, notwithstanding the low absolute concentrations, to account for the accumulation of such richly diatomaceous sediments, given a mechanism, namely upwelling, which continuously supplies dissolved nutrients to the euphotic zone. Recourse to volcanic sources of silica is then no longer necessary, in spite of clear stratigraphic associations, assumed to be genetic, between diatomaceous sediments and volcanic rocks.

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