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| Titel |
Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia? |
| VerfasserIn |
C. Lenz, T. Jilbert, D. J. Conley, M. Wolthers, C. P. Slomp |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 16 ; Nr. 12, no. 16 (2015-08-19), S.4875-4894 |
| Datensatznummer |
250118064
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| Publikation (Nr.) |
 copernicus.org/bg-12-4875-2015.pdf |
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| Zusammenfassung |
| Expanding hypoxia in the Baltic Sea over the past century has led to the
development of anoxic and sulfidic (euxinic) deep basins that are only
periodically ventilated by inflows of oxygenated waters from the North Sea.
In this study, we investigate the potential consequences of the expanding
hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of
pore water and sediment analyses of dated sediment cores from eight locations.
Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic,
hypoxic and euxinic sites are consistent with an active release of Mn from
these areas. Although the present-day fluxes are significant (ranging up to
ca. 240 μmol m−2 d−1), comparison to published water
column data suggests that the current benthic release of Mn is small when
compared to the large pool of Mn already present in the hypoxic and anoxic
water column. Our results highlight two modes of Mn carbonate formation in
sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely
form from Mn oxides that are precipitated from the water column directly
following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate
and Mn sulfide layers appear to form independently of inflow events, and are
possibly related to the much larger and continuous input of Mn oxides linked
to sediment focusing. Whereas Mn-enriched sediments continue to accumulate in
the Landsort Deep, this does not hold for the Gotland Deep area. Here, a
recent increase in euxinia, as evident from measured bottom water sulfide
concentrations and elevated sediment molybdenum (Mo), coincides with a
decline in sediment Mn content. Sediment analyses also reveal that recent
inflows of oxygenated water (since ca. 1995) are no longer consistently
recorded as Mn carbonate layers. Our data suggest that eutrophication has not
only led to a recent rise in sulfate reduction rates but also to a decline in
reactive Fe input to these basins. We hypothesize that these factors have jointly
led to higher sulfide availability near the sediment–water interface
after inflow events. As a consequence, the Mn oxides may be reductively
dissolved more rapidly than in the past and Mn carbonates may no longer form.
Using a simple diagenetic model for Mn dynamics in the surface sediment, we
demonstrate that an enhancement of the rate of reduction of Mn oxides is
consistent with such a scenario. Our results have important implications for
the use of Mn carbonate enrichments as a redox proxy in marine systems. |
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