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Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia?

Lenz, Conny LU ; Jilbert, T.; Conley, D. J.; Wolthers, M. and Slomp, C. P. (2015) In Biogeosciences 12(16). p.4875-4894
Abstract
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 mu mol m(-2)d(-1)), comparison to published water column data... (More)
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 mu mol 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. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Biogeosciences
volume
12
issue
16
pages
4875 - 4894
publisher
Copernicus Publications
external identifiers
  • wos:000360294800002
  • scopus:84939818245
ISSN
1726-4189
DOI
10.5194/bg-12-4875-2015
language
English
LU publication?
yes
id
0f1dd903-2abe-4d01-aeb3-3a993cf1788e (old id 7969186)
date added to LUP
2015-09-23 07:10:41
date last changed
2017-11-05 03:11:15
@article{0f1dd903-2abe-4d01-aeb3-3a993cf1788e,
  abstract     = {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 mu mol 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.},
  author       = {Lenz, Conny and Jilbert, T. and Conley, D. J. and Wolthers, M. and Slomp, C. P.},
  issn         = {1726-4189},
  language     = {eng},
  number       = {16},
  pages        = {4875--4894},
  publisher    = {Copernicus Publications},
  series       = {Biogeosciences},
  title        = {Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia?},
  url          = {http://dx.doi.org/10.5194/bg-12-4875-2015},
  volume       = {12},
  year         = {2015},
}