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要旨:
Iron (Fe) is an essential micronutrient and its availability controls
primary production in large parts of the ocean. Further, the variability
of oxygen minimumzones (OMZ) exerts a major control on the availability
of Fe, concentrations of which are elevated off Namibia. Therefore, to
understand the sources and processes involved in the cycling of Fe off
Namibia, we determined major and trace element contents (Fe, Al, Si, K,
P, organic carbon, Mn, Co, Zr) and the isotopic composition of Fe (delta
Fe-56) in seven sediment cores along a transect from the shelf to the
abyssal plain at similar to 5 km water depth (Meteor cruise M76-1) as
well as in the local lithogenic background (Namibian river bed sediments
and dust).
The depletion of Mn vs. the local lithogenic background (LLB) reflects
the reducing character of sediments from the shelf to the lower slope.
While depletion of Mn and enrichment of Co indicates the anoxic-sulfidic
environment on the shelf, the enrichment of both elements in abyssal
sediments documents highly oxidizing conditions there.
Iron/Al ratios are elevated (0.6 to 1.6) and therefore equal to, or
higher than the LLB. The highest Fe enrichment is seen on the shelf
(core1) and the shelf break (core2) with a clear decrease towards the
abyss. The bulk delta Fe-56 values range from +0.30 to -0.05 parts per
thousand in the marine sediments, with an average of similar to 0.08
parts per thousand, which is slightly lighter than the delta Fe-56 of
the LLB (0.16 to 0.21 parts per thousand). Further, combined enrichments
of K and Fe suggest the presence of detrital biotite (core 1) and
authigenic glauconite (core2), both of which are perfectly correlated
with Zr/Al ratios reflecting the highly dynamic conditions of the shelf
environment (0-300 m water depth).
The absent large sedimentary Fe isotope fractionation in the sediments
of this study compared to Peru sediments is likely due to a combination
of the following: (i) a highly variable OMZ leading to random
re-oxidation of any sediment-sourced Fe which prevents the export of
larger amounts of dissolved Fe to the interior of the ocean, and which
also prevents focused enrichments of Fe, and (ii) an enhanced dust flux,
which frequently resupplies terrestrial Fe dampening any Fe isotope
fractionation.
This study is a snapshot on sedimentary Fe cycling in one area from the
vast Namibian margin. Compared to the Peruvian margin, the data suggest
little sedimentary Fe release, transport and redeposition. However,
previous water column data suggest substantial Fe release. Hence, for a
more subtle investigation of the Fe loss and re-supply a higher sampling
resolution of multiple cores across the shelf and slope will be
required. (C) 2020 Elsevier B.V. All rights reserved.
