Mineralogy and Diagenesis of Surface Sediments from DOMES Areas A, B, and C
Box cores were collected between 10 to 15°N latitude and 126 to 151°W longitude in the North Pacific. Sediments are primarily siliceous fossil rich mud, but span the range from siliceous ooze to red clay; there is also one bed of early Miocene nannofossil ooze. Terrigenous debris consist of quartz, feldspar, illite, and chlorite + kaolinite; andesitic volcanic glass shards, biotite, and other associated volcanic materials are also present in minor amounts and probably originated from Central and South American explosive volcanism. In general, terrigenous minerals decrease in abundance seaward and in pre-Quaternary deposits.
Siliceous microfossils — radiolarians, diatoms, and silicoflagellates — are the most abundant biogenic component, up to 50% by volume. These microfossils undergo extensive dissolution within the upper one-half meter of sediment. Spicules, the most robust siliceous microfossil, are the only forms remaining in the lower parts of most cores. Superimposed on the overall gradual decrease in siliceous fossils with depth, which is the result of dissolution, is a cyclic fluctuation in siliceous debris; the cyclic variation may result from changes in productivity in surface waters. Previous studies show that dissolution of biogenic opaline silica releases Si, Al, Cu, Ni, and perhaps Mn and Zn which in DOMES deposits are rapidly incorporated into authigenic smectite, zeolites, and ferromanganese nodules. Other biogenic components are nannofossils, fragments of planktic and benthic foraminifers, fish debris, fragments of worm tubes, and fecal pellets.
Authigenic minerals are important sediment builders in the equatorial North Pacific. Authigenic Fe-rich smectite, on the average, makes up 10% of the sediment. Minor authigenic barite, apatite, hematite, clinoptilolite, phillipsite, and perhaps atacamite, also occur in DOMES cores. The smectite most likely forms from the chemical combination of Fe oxyhydroxide and Si at low temperatures. The Fe oxyhydroxide is probably derived from volcanic activity on the East Pacific Rise and is dispersed in colloidal form throughout the Pacific. We suggest that the Si, Al, Cu, Ni, and Zn in the smectite are derived from the dissolution of biogenic silica and from displacement of minor elements adsorbed on Fe-Mn oxyhydroxides.
Formation of Fe-rich smectite may be the mechanism that fractionates Fe and Mn (combined in oxyhydroxides) in the sediment and nodule phases respectively. Silica may be the controlling constituent. In areas of siliceous ooze, Fe smectite forms relatively rapidly and, consequently, Mn nodules in the same deposits have high Mn/Fe ratios. In contrast, nodules formed in red clay areas have low Mn/Fe ratios possibly because of the relatively limited silica supply.
Locally abundant metalliferous sediment (deposits enriched in Fe, Mn, Cu, and Ni) probably formed by the extensive dissolution of biogenic debris. Dissolution would leave a metalrich residue whose position may be useful for paleo-oceanographic studies.
Minor authigenic atacamite, barite, apatite, hematite, clinoptilolite, and phillipsite also occur in DOMES cores.
KeywordsVolcanic Glass Ferromanganese Nodule Metalliferous Sediment Siliceous Microfossil Dome Area
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