MINERALOGY, GEOCHEMISTRY, AND ORIGIN OF PACIFIC RED CLAYS - A REVIEW

Pacific red clays are dominantly allogenic in origin. The allogenic component is derived principally from aeolian dust from central Asia, Australia, and Central America. The highest input of dust is from central Asia. Sedimentation rates of red clays are therefore higher in the North Pacific than in the South Pacific. Being composite in origin, the allogenic fraction of red clays is similar in composition to that of shales. However, this does not necessarily imply a genetic origin of red clays from shales. Sedimentation of red clays is variable through time. The highest rates of allogenic sedimentation were during glacial periods when aeolian dust transport was a maximum. Red clays are found in oxic pelagic environments where reduction of nitrate by organic carbon in the pore waters does not occur. This permits oxidation of Mn and Fe to their higher valencies with the consequent adsorption of certain transition elements on their oxides. This situation corresponds to sedimentation rates of < 40 mm/10(3) yr. Accumulation rates of authigenic elements (Mn, Co, Cu, and Ni) are inversely related to sedimentation rate. This explains the higher contents of these elements in South Pacific red clays compared to those from the North Pacific. About 90% Mn, 80% Co and Ni, and 50% Cu in red clays are of authigenic origin. The diffusive flux of Mn in red clays is small and corresponds to about 7% of the total sedimentation rate of Mn. By contrast, 96% of Cu in red clay is regenerated in the sediment. Hydrothermal input of elements to sediments falls off rapidly with distance from the crest of mid-ocean ridges. This explains the small component of hydrothermally derived elements in Pacific red clays.