Growth of a manganese nodule from Peru Basin: A radiochemical anatomy

Attempts have been made to study the entire growth history of a manganese nodule from the northern part of Peru Basin in the Pacific using radiochemical profiles of 230Th232Th, 227Th230Th, and 10Be9Be. Combined with the observations on Fe-Mn contents and textural variation, the radiochemical data indicate that the nodule grew more or less concentrically throughout most of its existence since it formed 1.5 my ago, receiving Mn from both bottom water and pore water. This condition appeared to have changed about 180 ky ago when the growth became asymmetric in that the top and bottom sides became fixed in their relative positions on the sea floor. Since then, the bottom side accreted with a fast rate of close to 200 mm/my, apparently fueled by the supply of diagenetically remobilized Mn in pore water from the sediment substrate. In the meantime, the top side accumulated at about 6 mm/my, a value which is in the normal range for deep-sea nodules having their Mn supplied from the hydrogenous source.