Iron isotope and trace metal compositions of manganese nodules from the
Central Indian Basin: Evidence for Fe fluxes from both hydrogenetic and
oxic diagenetic sources
Abstract
Iron isotope compositions (δ56Fe) of ferromanganese
(Fe-Mn) crusts and nodules are reliable proxies for understanding the
biogeochemical cycling of Fe in the ocean. Fe-Mn nodules/crusts are
characterized by low δ56Fe values (–0.8 to –0.05 ‰
IRMM 014), while the dissolved Fe in the deep ocean is
56Fe enriched (d56Fe range from +0.2
to +0.8 ‰). Here, we report Fe isotope compositions of top scrapings of
sixteen Mn nodules and three Fe-Mn crusts and their geochemical
compositions from the Central Indian Basin (CIB) to understand Fe
isotope compositions. Based on their morphology and elemental
compositions, the CIB nodules are divided into three groups:
‘hyrogenetic,’ ‘diagenetic,’ and ‘mixed’ types. The range of
δ56Fe values (-0.63 to -0.06 ‰) for CIB nodules and
crusts is similar to those from different parts of the world ocean. The
δ56Fe values of the hydrogenetic group of CIB nodules
are consistent with a fractionation model involving selective adsorption
56Fe onto organic ligands (siderophore complexes).
Using a fractionation factor of -0.77 ‰ between seawater and nodules, we
estimate that δ56Fe of the CIB deep seawater dissolved
Fe range between +0.28 and +0.63 ‰ similar to the Atlantic deep
seawater. The δ56Fe values of mixed nodules correlate
positively with Mn/Fe and concentrations of Mn, Cu, Zn, Mo, Cd, Sb, and
Tl, and negatively with Fe, Be, Sc, Co, Zr, Nb, and rare earth elements
(REE). These mixing lines attest to variable proportions of metals from
diagenetic and hydrogenetic sources.