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| Titel |
Enrichment of mobilizable manganese in deep sea sediments in relation to Mn nodules abundance |
| VerfasserIn |
Philipp Widmann, Thomas Kühn, Hartmut Schulz |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089666
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| Publikation (Nr.) |
 EGU/EGU2014-3875.pdf |
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| Zusammenfassung |
| The Clarion-Clipperton-Fracture Zone located in the northern Central Pacific is
a promising area for future Manganese nodules mining. Yet, nodule abundance
varies within tens of meters within a prospective field. However, economic nodule
mining requires a detailed knowledge of nodule distribution and its controlling
factors.
In order to examine factors controlling occurrence, size, composition, and formation of
ferromanganese nodules, selective leaching experiments of sediments from 37 box cores were
carried out. Data was compared to XRF analysis of the bulk composition of the sediments,
grain size distribution, scanning electron microscope (SEM) analysis of sedimentary
components and total organic carbon measurements. To interpret local variations of the
sediment geochemistry other influencing factors have been taken into account such as
hydrodynamic conditions, seafloor topography and sedimentation rates of the sampling
sites.
Due to recent oxic conditions at the seafloor manganese is accumulated in its insoluble
oxidized form (Mn4+) and nodules grow slowly and predominant by hydrogenetic
precipitation. However, suboxic diagenesis prevailed periodically in the past during which
Mn4+ got mobilized, leading to enhanced nodule growth and depletion of Mn within the
sediment. First results of our leaching approach indicate that more than 80% of the total
manganese is bound as Mn4+-oxide in the near surface sediments decreasing with depth to
~50 % in average. Base metals such as Ni and Co are strongly associated with Mn therefore
following the same leaching pattern. However, under the conditions of suboxic
diagenesis Fe is not mobilized. Different types of Mn4+ / Mntotal and Corg profiles can
be distinguished and correlated to nodule abundance, even if concentration-depth
profiles of Corg, Mn4+ and Mntotal are highly variable. However, some prominent
features are common varying with depth, probably indicating variable hydrodynamic
conditions.
In general, cores without or with very low nodule abundance show low Mntotal
concentrations, a constant decrease of the total and mobilizable Mn4+ with depth and no or
only slight degradation of Corg, indicating oxic conditions. In contrast, cores with a high
coverage are in general characterized by large nodules, high Mntotal concentration and a
strong depletion of mobilizable and Mntotal within the upper cm, primarily followed by an
increase or steady concentration.
Cores from the south-eastern part of the research area show a pronounced carbonate peak at
19-28 cm bsf. which may correlate to the last glacial maximum (LGM). SEM analysis
indicates a high content of well preserved foraminifers. Within this layer no Mn micro
nodules are found. However, micro nodules are found above this layer associated with a
strong increase of leachable Mn4+ and siliceous microfossils. Therefore, it is assumed that
the last major mobilization event took place at the LGM. Increasing abundance of micro
nodules towards the top indicate oxidized top layers and fixation of mobilized Mn2+
migrating upwards.
Our data indicate that nodule abundance depends on the combination of mobilizable Mn4+
concentration, Mntotal, Corg concentration and degradation as well as mobilization intensity.
However, local variations may be caused by hydrodynamic conditions, sediment properties
and the intensity of sediment accumulation or erosion. |
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