Growth of diagenetic ferromanganese nodules in an oxic deep-sea sedimentary environment, northeast equatorial Pacific

To study the accretion mechanism of diagenetic ferromanganese nodules forme d in an oxic deep-sea sedimentary environment, manganese nodules, sediments , and pore waters were sampled in the northeast equatorial Pacific. Diagene tic ferromanganese nodules are ellipsoidal to discoidal in shape, rough in surface texture, enriched in Cu, Ni and 10 Angstrom-manganite, and are ubiq uitous on the abyssal plain; hydrogenetic nodules are polynucleated, irregu lar in shape, smooth in surface texture, enriched in Co and delta-MnO2, and occur dominantly on seamounts. Numerous microlayers with diagenetic or hyd rogenetic characteristics occur successively in the nodules. Concentration of nitrate in sediment pore waters indicates that overall redox conditions are oxic. However, Mn-oxide grains on the walls of burrows and worm tubes a re interpreted to be reprecipitated metal ions supplied from oxygen-deplete d microenvironments that formed in the oxic sediment column by decompositio n of organic matter. Active mixing of bottom sediments by benthic fauna and bottom currents is indicated by ubiquitous burrows, worm tubes, and tracks and trails, high values in sediment mixing coefficients (>1 cm(2)/yr), and scattering of data points in depth profiles of pore water nitrate concentr ations. We suggest that diagenetic ferromanganese nodules in an oxic deep-s ea sedimentary environment grew from remobilized metal ions as well as repr ecipitated Mn-oxide grains, which were supplied to the nodules episodically during the stirring of bottom sediments by benthic fauna and intermittent strong bottom current flow. (C) 1999 Elsevier Science B.V. All rights reser ved.