Compositional variation in Pacific Ocean ferromanganese nodules and its relationship to sediment accumulation rates

Abstract

Recent work on the geochemistry of deep- and shallow-water manganese nodules, and the post-depositional migration of manganese and related metals in marine sediments, has emphasized the role of diagenesis in the formation of manganese nodules and concretions. These observations have led to a reappraisal of the contrasting compositions and modes of formation of open oceanic and shallow marine nodules. The Mn Fe ratio in Pacific Sea floor nodules is highest along the eastern margin and lowest in the southwest central region. The Fe Co and Mn Ni ratios are also highest in the eastern Pacific but lowest in the north and south central Pacific. The enrichment of Mn in marginal nodules is caused by the diagenetic migration of Mn within the Recent sediments and its precipitation at the sediment surface. The Mn Fe ratio is therefore a useful index of diagenetic derivation of metals in nodules. The depletion of minor metals in marginal nodules, in spite of an adequate source, must similarly be controlled by diagenetic processes within the sediments. Since diagenesis is largely governed by the amount and type of organic matter in sediments and this depends on the rate of sediment accumulation, diagenesis is most marked in marginal, rapidly accumulating sediments and minimal in open oceanic sediments. The regional variations in nodule composition therefore reflect the broad pattern of sedimentation in the Pacific, an explanation which is at variance with the current view that volcanism controls the composition and the formation of manganese nodules.