Palaeoceanographic conditions during the formation of a ferromanganese crust from the Afanasiy-Nikitin seamount, North Central Indian Ocean: geochemical evidence

A ferromanganese crust dredged from the summit of the Afanasiy-Nikitin seamount in the North Central Indian Ocean (NCIO) has recorded Neogene oceanographic events. The substrate of the crust is composed of fresh-water phreatic calcite cement, Terebratulinae casts, rounded and ferruginised basalt clasts and weathered coralline algal fragments suggesting subaerial exposure during the Oligocene (~30 Ma) global sealevel drop. The mineralogy, major, trace and REE element geochemistry and Co-model age estimates suggest three distinct accretionary environments during the crust growth: (1) a period of contemporary precipitation of Fe-Mn oxide and carbonate fluorapatite (CFA) indicate an intensified intermediate water oxygen minimum zone (OMZ) in the late Miocene, (2) a pulse of very high CFA deposition and detrital input in addition to Fe-Mn oxide accretion at the close of Miocene reflect a more intense OMZ and the erosion of the Himalayas and, (3) more oxidizing conditions of the ambient seawater due to contraction of the late Miocene OMZ facilitated the accretion of almost pure Fe-Mn oxide since the Pliocene. These interpretations solely depend upon the chronology based on an empirical relationship and are subjected to the confirmation by radiometric dating. As a consequence of significant deposition of CFA in addition to Fe-Mn oxide during the intensified OMZ, the normally coherent behaviour of trivalent rare earth elements (3+ REE) is not observed in the crust. La, Yb and Lu show a positive association with the CFA phase elements (Ca and P) and Pr, Nd and Sm with the Mn-oxide phase elements (Mn, Co and Ni). This leads to an abnormal, incoherent behaviour of the 3+ REE in multimineral authigenic system.