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| Titel |
Seasonal and event-driven changes in the cycling and isotopic composition of molybdenum in tidal systems of the North Sea |
| VerfasserIn |
Nicole Kowalski, Olaf Dellwig, Nadja Neubert, Ulf Gräwe, Melanie Beck, Thomas F. Nägler, Hans-Jürgen Brumsack, Justus E. E. van Beusekom, Michael E. Böttcher |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053454
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| Zusammenfassung |
| Seasonal dynamics of the redox-sensitive trace metal molybdenum (Mo) and its stable
isotopic composition were investigated in the water column and surface sediments of two
tidal systems of the southern and northern parts of the German Bight: The backbarrier area of
Spiekeroog Island and the tidal basin of Sylt Island.
Mo is generally assumed to behave conservatively in oxic seawater following solely
changes in salinity. In recent years, however, decreases in Mo concentration as low as 50 % of
normal values were observed during certain periods in summer without accompanying
changes in salinity. This non-conservative behaviour was recognized repeatedly in the water
column of the tidal areas of the North Sea. The corresponding δ98-95Mo values show a trend
towards a heavier isotope composition in the residual Mo fraction indicating isotopic
fractionation during the depletion period and the removal of a light Mo isotope fraction from
the aqueous phase. As this period coincides with summer phytoplankton blooms we suggest a
tight coupling to algae derived organic matter and transport of Mo into the sediments by
organic-rich aggregates. After burial, these aggregates are rapidly decomposed by
microorganisms and Mo is released to the pore water. In agreement with this, pore waters are
enriched in the lighter Mo isotope compared to the water column. An interesting
observation was made regarding the Mo isotopic composition of the exterior organic
layer (periostracum) of the shell of the invader mussel Ensis americanus. Distinct
enrichments of the lighter isotope and max contents of 160 mg kg-1 Mo were
observed thus supporting a possible association of Mo to organic matter in the Wadden
Sea.
By contrast, significant enrichments of dissolved Mo were observed in the water column
of the Sylt area in early summer 2009, which is likely attributed to benthic-pelagic cycling,
e.g., Mo liberation from (suboxic) reduction of sedimentary Mn oxides.
Laboratory experiments with anoxic sediments demonstrate that Mo is efficiently
released during sediment re-suspension in oxic waters. Therefore, intense sediment
re-suspension by high-energy bottom currents and in particular during storm events likely
causes effective oxidative release of Mo, thereby accelerating benthic-pelagic Mo
cycling. The experimental data will be used in a modelling approach, which simulates
sediment re-suspension to estimate the importance of these processes on Mo cycling. |
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