Growth History and Variability of Manganese Nodules of the Equatorial North Pacific

Abstract

“Seeding” of coarse particles at marked horizons is responsible for the origin of manganese nodule fields. Such “seeding” occurred at hiatuses which were formed by the erosional capacity of the currents of the Antarctic Bottom Water during the Neogene. Two kinds of nodules are prevalent, one of which owes its existence to these hiatuses. This is the main type of the equatorial North Pacific. The other nodule type originated at the surface of consolidated and bioturbated ash layers. A steady process of biogenic lifting has prevented most of the nodules from burial. Occasionally encountered buried nodules prove, however, that nodules form at a distinct horizon, as well as reveal their progressive growth stages. Pelagic accumulation rates are directly related to the rate of decay of organic matter within near-surface sediments. This decay correlates positively with the Mn-flux within the interstitial water in seafloor sediments. The Mn-flux, in turn, is responsible for the diagenetic growth of nodules. The amount of organic matter available on the seafloor positively influences how intense the activity of burrowing organisms is, and it is this activity that keeps nodules from being buried below the sedimentary surface. Sediment accumulation rates are mainly determined by the velocity of bottom currents, which, in turn, are strongly influenced by bottom morphology. Mn-nodule deposits in the equatorial North Pacific Ocean represent accumulations at least during the last 15 million years. From site to site, however, sedimentation rates, geochemical and oceanographic conditions differ, and this may explain the great variability of manganese nodule facies.