EVIDENCE FOR EARLY DIAGENETIC MOBILIZATION OF TRACE-METALS FROM DISCOLORATIONS OF PELAGIC SEDIMENTS

From two core sections of Pacific Ocean pelagic sediment yellowish colored spots and surrounding brown sediments were analyzed for trace metals. In addition, a dark-brown sample with a high trace-metal content was subjected to fractional extraction with a mild acidic and reducing solution. In both the naturally discolored spots and the experimental extraction, Mn, Ni and Co were simultaneously mobilized, whereas Cu was mobilized to a considerably lesser extent. Fe remained almost completely unaffected. From these observations it can be concluded: (1) that in these sediments Ni and Co occur in an intimate mixture or in solid solution with the manganese oxides, while Cu and probably Zn seem to be bound to other mineral phases either initially in the brown-colored sediments or immediately after release from Mn oxides; (2) that slightly reducing conditions existed within micro-areas of these highly oxidizing sediments for a limited period of time during their early diagenetic history, mobilizing a large fraction of those trace metals which are preferentially found in Mn nodules at the sediment surface, and (3) that the brown color of these pelagic sediments is caused exclusively by Mn oxides, while Fe is present in more or less colorless compounds. © 1979.