The formation of manganese nodules between the Clarion and Clipperton fracture zones southeast of Hawaii

Abstract

Three areas in the equatorial North Pacific between the Clarion and Clipperton fracture zones were investigated regarding depositional and diagenetic processes during manganese nodule formation. The areas lie on a NW-SE trending trace of the Pacific plate southeast of Hawaii. The manganese nodule facies shows low nodule abundance and low-grade (Cu+Ni+Co) nodules in the northwest but high abundance and high grade nodules in the southeast. The abundance, size, structure and composition of the nodules are determined by the nature of the underlying sediment. Breaks in sedimentation during the Oligocene and Miocene play a highly significant role in the formation of manganese nodules. This was the time when the growth of most nodules began, favoured by an abundance of potential nuclei. Ample supply of organic matter and of skeletal material was important for the diagenetic growth of nodules rich in Cu, Ni and Co. A large amount of organic matter stimulated bioturbation and hence lifting by benthic organisms, which in turn kept the nodules on top of the sediment. Even during the post-hiatus periods, the two areas in the southeast were always located in a zone of high biological productivity and hence much organic matter was supplied to the seafloor. As a consequence, most of the fields containing a great abundance of nodules rich in Cu, Ni and Co are found there. Due to the northwestward motion of the Pacific plate the northwestern area has been located, at least since the Early Miocene, outside the region with such favourable conditions. Nodules of low abundance and poor in Cu, Ni and Co are dominant. In spite of this distinct differentiation into two major or regional nodule facies areas, each area within itself is very complex. The development of the minor or local nodule facies is determined mainly by the position of the nodules in relation to the calcite compensation depth, the post-hiatus sediment thickness, the degree of reworking, the potential for formation of nuclei during hiatus periods and the rate of supply or decomposition of organic matter. These factors are in turn strongly controlled by the morphology of the seafloor and the activity of bottom currents.