Occurrence of manganese nodules of GH81-4 survey area

Local variability of manganese nodule facies and associated sediments were studied around abyssal hills (60 x 60 km²) located in the equatorial zone of the Central Pacific Basin. Small-scale variations in nodule occurrence, mineralogy, chemistry, and internal structure are discussed in relation to seismic records and sediment lithology. The slow sedimentation rate from the late Tertiary to the Quaternary appear to have promoted formation of nodules around the abyssal hills, although the Plio-Pleistocene hiatus at depths below sediment surface suggests no clear evidence of increased growth of nodules during the period. In the area of rapid and continuous sedimentation away from the hill areas, nodules are rare probably because of insufficient lifting force for nodules. Mineralogical and chemical studies reveal that the two principal nodule morphologies (smooth and rough surfaces) result from preferential deposition of hydrogenous or diagenetic manganese minerals in relation to early diagenesis of surface siliceous sediments. Regional variability of nodule-surface morphology and composition suggests a very local change in sedimentary conditions in space, and internal mineral variation suggests changes in sedimentary conditions with time. The geological factors controlling the small-scale variability of nodule facies seem to be similar to those controlling regional-scale variations.