Transition metal and rare earth element fluxes at two sites in the eastern tropical Pacific : relationship to ferromanganese nodule genesis Public Deposited

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  • Transition metal and rare earth element (REE) data are presented for various sedimentary materials from two Manganese Nodule Program (MANOP) sites in the eastern tropical Pacific. These data constrain the sources of elemental supply to the seafloor and the processes that control ferromanganese nodule genesis. Investigation of elemental partitioning between suspended particles, sediments, ferromanganese nodules, crusts, and micronodules indicates that particle settling dominates REE supply to the seafloor at MANOP site H. Partitioning of the particle flux reveals that although detrital and labile particles both convey REE to H sediments and nodules, labile biogenic debris is the primary source of REE accreted by site H nodules. Evidence from REE abundance patterns and correlations of REE with other elements define three basic processes of nodule accretion: (1) hydrogenous or direct seawater precipitation; (2) oxic diagenesis, referring to a variety of ferromanganese accretion processes which occur in oxic sediments; and (3) suboxic diagenesis, in which Mn is mobilized by the oxidation of sedimentary organic matter. The REE geochemistry of site H nodules suggests an additional accretionary process, resulting from the degradation of labile organic matter in an oxic environment. Contrasts in REE patterns, major element compositions, and fluxes of various elements in sedimentary samples from MANOP site H (a nodule-bearing site) with site M (a nodule-free site) reveal differences in the sources of material to the seafloor in the two environments. Geochemical data imply a greater hydrothermal influence and a greater accumulation of organic carbon in site M sediments relative to those at site H. Comparisons of the REE and major element geochemistry of H and M sediment traps, sediments, and crusts, as well as nodules from site H, imply that the reductive mobilization of Mn and geochemically similar elements is controlled primarily by surface water biological productivity and the accumulation of organic carbon in the sediments. The organic flux to the sediments determines the degree of depletion of oxidants, which governs the reduction, mobilization, and supply of metals to nodule surfaces. It appears that the depth to the Mn redox boundary and possibly seasonal variations in the flux of organic material to the seafloor determine whether nodules grow or whether Mn is exported to bottom waters.
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