Manganese nodule morphology as indicators for oceanic processes in the Central Indian Basin

Abstract

In order to understand the role of geological features in the depositional environment and the prevailing oceanic processes on the formation and characteristics of manganese nodules, a detailed morphological study of the manganese nodules was undertaken on 23,000 nodules from 194 locations (including 801 substations) in a nodule-rich area covering about 150,000 km sup(2) in the Central Indian Basin (CIB). Nodules with rough surface texture dominate most of the area except the south-eastern part of the basin, which is floored more by the smooth nodules. Smaller nodules (is less than 4 cm) are common and are dominant both in density and mass in the south-eastern part of the basin, whereas the north-western part and the central part show dominance of larger rough nodules with higher density and mass. Smooth nodules are also found at shallower depth (less than 5000 m), on the seamount tops and along the slopes, whereas the rough nodules mostly occur in deeper areas. Significantly, the eastern part of the basin show smooth nodules with smaller size. Smooth nodules is greater than 4 cm diameter are rare and show low oxide layer thickness and low bio-sediment remnants compared to rough surfaced nodules. Large variation in morphological types of nodules are found in the CIB with spheroidal, oblong, triangular, rounded, sub-rounded or irregular shapes, with irregular nodules being most common. The most common nucleus is altered basalt, while pumice, shark teeth, clay and older nodule nuclei are also present. Water currents and seafloor topography seem to play a major role in defining the nodule morphology. Results of the study show the abundance of smaller nodules with smooth surface texture towards the eastern side of the study area. These features are probably responding to bottom current activity. Inasmuch as the eastern part of the study area is closer to saddles in the Ninety East Ridge (which is the entry point of the Antarctic Bottom Water (AABW) currents into the CIB), the influence of AABW is reflected in the shape and size of the nodules in this area