Implications of REE incorporation and host sediment influence on the origin and growth processes of ferromanganese nodules from Central Indian Ocean Basin

Abstract

This study presents new major, trace and REE data for thirty-five ferromanganese nodules recovered from areas representing three different sediment types (siliceous, red clay and their transition zone) in the Central Indian Ocean Basin (CIOB) to address their genetic aspects, classification, growth rate, nature of host sediments and influence of REE in the processes of nodule formation. The nodules from CIOB are mostly either hydrogenetic (metals coming from oxygenated bottom water) and diagenetic (metals coming from suboxic sediment pore water) or a combination of both, depending on the source of supply of metal. However, a number of biogeochemical processes mediate this supply of metals which again changes from time to time, making the nodule growth process highly dynamic. This study suggests that at the initial stage of nodule growth, host sediments do not play much role in controlling the growth processes for which REEs can enter both Mn and Fe oxyhydroxide phases equally. Thus, the bottom water signature is imprinted in these early formed nodules irrespective of their host sediment substrate but with gradual growth and burial in the sediment, the main mode of metal enrichment becomes diagenetic through sediment pore water. This tends to increase the concentration of Mn, Ni and Cu over other elements which are retained in the sediment fraction. Among the REEs, Ce concentration of the nodules shows significant positive anomaly due to variation in redox potential and hence its magnitude can be used to get an idea about the metal enrichment procedure and the genetic type of the nodules. However, based on host sediment only, not much difference is found in the magnitude of Ce anomaly in these nodules. On the other hand, discrimination diagram, based on HFSE and REY chemistry, indicates that most of these nodules are of diagenetic origin under oxic condition with a trend towards hydrogenetic field. Further, the genetic type of the ferromanganese nodules from the CIOB are more effectively differentiated by a combination of their major and trace element concentrations rather than solely based on their REE or HFSE chemistry or host sediment substrate