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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
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  • Sreenivas Bulusu,
  • Bejugam Nagender Nath,
  • Y.J. Bhaskar Rao,
  • B Vijaya Gopal,
  • T.C. Vineesh,
  • E.V.S.S.K. Babu
Sreenivas Bulusu
CSIR-National Geophysical Research Institute

Corresponding Author:bsreenivas@ngri.res.in

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Bejugam Nagender Nath
National Institute of Oceanography
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Y.J. Bhaskar Rao
CSIR-National Geophysical Research Institute
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B Vijaya Gopal
CSIR-National Geophysical Research Institute
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T.C. Vineesh
CSIR-National Institute of Oceanography
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E.V.S.S.K. Babu
CSIR-National Geophysical Reserch Institute
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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.