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

Abstract. Expanding hypoxia in the Baltic Sea over the past century has led to 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 consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of well-dated sediment cores from 8 locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic and hypoxic sites are in line with an active release of Mn from these areas. However, this flux of Mn is only 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 form independent of inflow events, with pore water Mn produced in deeper layers of the sediment acting as a key Mn source. While formation of Mn enrichments in the Landsort Deep continues to the present, this does not hold for the Gotland Deep area. Here, increased euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), goes hand in hand with a decline in sediment Mn and recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. We postulate that the reduction of Mn oxides by hydrogen sulfide following inflows has become so rapid that Mn2+ is released to the water column before Mn carbonates can form. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems.

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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?

Biogeosciences ◽

10.5194/bg-12-4875-2015 ◽

2015 ◽

Vol 12 (16) ◽

pp. 4875-4894 ◽

Cited By ~ 30

Author(s):

C. Lenz ◽

T. Jilbert ◽

D.J. Conley ◽

M. Wolthers ◽

C.P. Slomp

Keyword(s):

Baltic Sea ◽

Water Column ◽

North Sea ◽

The Baltic Sea ◽

The Past ◽

Gotland Deep ◽

Carbonate Formation ◽

The Baltic ◽

Mn Oxides ◽

Deep Area

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 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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Bacteriohopanepolyols record stratification, nitrogen fixation and other biogeochemical perturbations in Holocene sediments of the central Baltic Sea

Biogeosciences ◽

10.5194/bg-10-2725-2013 ◽

2013 ◽

Vol 10 (4) ◽

pp. 2725-2735 ◽

Cited By ~ 30

Author(s):

M. Blumenberg ◽

C. Berndmeyer ◽

M. Moros ◽

M. Muschalla ◽

O. Schmale ◽

...

Keyword(s):

Baltic Sea ◽

Water Column ◽

Stable Stratification ◽

The Baltic Sea ◽

Allochthonous Input ◽

The North ◽

Water Column Stratification ◽

Gotland Deep ◽

History Of ◽

The Baltic

Abstract. The Baltic Sea, one of the world's largest brackish-marine basins, established after deglaciation of Scandinavia about 17 000 to 15 000 yr ago. In the changeable history of the Baltic Sea, the initial freshwater system was connected to the North Sea about 8000 yr ago and the modern brackish-marine setting (Littorina Sea) was established. Today, a relatively stable stratification has developed in the water column of the deep basins due to salinity differences. Stratification is only occasionally interrupted by mixing events, and it controls nutrient availability and growth of specifically adapted microorganisms and algae. We studied bacteriohopanepolyols (BHPs), lipids of specific bacterial groups, in a sediment core from the central Baltic Sea (Gotland Deep) and found considerable differences between the distinct stages of the Baltic Sea's history. Some individual BHP structures indicate contributions from as yet unknown redoxcline-specific bacteria (bacteriohopanetetrol isomer), methanotrophic bacteria (35-aminobacteriohopanetetrol), cyanobacteria (bacteriohopanetetrol cyclitol ether isomer) and from soil bacteria (adenosylhopane) through allochthonous input after the Littorina transgression, whereas the origin of other BHPs in the core has still to be identified. Notably high BHP abundances were observed in the deposits of the brackish-marine Littorina phase, particularly in laminated sediment layers. Because these sediments record periods of stable water column stratification, bacteria specifically adapted to these conditions may account for the high portions of BHPs. An additional and/or accompanying source may be nitrogen-fixing (cyano)bacteria, which is indicated by a positive correlation of BHP abundances with Corg and δ15N.

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Bacteriohopanepolyols record stratification, nitrogen fixation and other biogeochemical perturbations in Holocene sediments of the Central Baltic Sea

Biogeosciences Discussions ◽

10.5194/bgd-9-17139-2012 ◽

2012 ◽

Vol 9 (12) ◽

pp. 17139-17165

Author(s):

M. Blumenberg ◽

C. Berndmeyer ◽

M. Moros ◽

M. Muschalla ◽

O. Schmale ◽

...

Keyword(s):

Baltic Sea ◽

Water Column ◽

Stable Stratification ◽

The Baltic Sea ◽

Allochthonous Input ◽

The North ◽

Water Column Stratification ◽

Gotland Deep ◽

History Of ◽

The Baltic

Abstract. The Baltic Sea, one of the world's largest brackish-marine basins, established after deglaciation of Scandinavia about 17 000 to 15 000 yr ago. In the changeable history of the Baltic Sea, the initial freshwater system was connected to the North Sea about 8000 yr ago and the modern brackish-marine setting (Littorina Sea) was established. Today, a relatively stable stratification developed in the water column of the deep basins due to salinity differences. Stratification is only occasionally interrupted by mixing events, and controls nutrient availability and growth of specifically adapted microorganisms and algae. We studied bacteriohopanepolyols (BHPs), lipids of specific bacterial groups, in a sediment core from the Central Baltic Sea (Gotland Deep) and found considerable differences between the distinct stages of the Baltic Sea's history. Individual BHP structures indicate contributions from as yet unknown redoxcline-specific bacteria (bacteriohopanetetrol isomer), methanotrophic bacteria (35-aminobacteriohopanetetrol), cyanobacteria (bacteriohopanetetrol cyclitol ether isomer) and, through allochthonous input after the Littorina transgression, from soil bacteria (adenosylhopane), whereas the origin of other BHPs in the core has still to be identified. Notably high BHP abundances were observed in the deposits of the brackish-marine Littorina phase, particularly in laminated sediment layers. Because these sediments record periods of stable water column stratification, bacteria specifically adapted to these conditions may account for the high portions of BHPs. An additional and/or accompanying source may be nitrogen-fixing (cyano)bacteria, which is indicated by a good correlation of BHP abundances with Corg and δ15N.

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Nitrous oxide water column distribution during the transition from anoxic to oxic conditions in the Baltic Sea

Biogeosciences Discussions ◽

10.5194/bgd-3-729-2006 ◽

2006 ◽

Vol 3 (3) ◽

pp. 729-764 ◽

Cited By ~ 3

Author(s):

S. Walter ◽

U. Breitenbach ◽

H. W. Bange ◽

G. Nausch ◽

D. W. R. Wallace

Keyword(s):

Nitrous Oxide ◽

Baltic Sea ◽

Water Column ◽

North Sea ◽

Deep Water ◽

Sea Water ◽

The Baltic Sea ◽

Gotland Basin ◽

Water Renewal ◽

The Baltic

Abstract. In January 2003, a major inflow of cold and oxygen-rich North Sea Water in the Baltic Sea terminated an ongoing stagnation period in parts of the central Baltic Sea. In order to investigate the role of North Sea Water inflow to the Baltic Sea with regard to the production of nitrous oxide (N2O), we measured dissolved and atmospheric N2O at 26 stations in the southern and central Baltic Sea in October 2003. At the time of our cruise, water renewal had proceeded to the eastern Gotland Basin, whereas the western Gotland Basin was still unaffected by the inflow. The deep water renewal was detectable in the distributions of temperature, salinity, and oxygen concentrations as well as in the distribution of the N2O concentrations: Shallow stations in the Kiel Bight and Pomeranian Bight were well-ventilated with uniform N2O concentrations near equilibrium throughout the water column. In contrast, stations in the deep basins, such as the Bornholm and the Gotland Deep, showed a clear stratification with deep water affected by North Sea Water. Inflowing North Sea Water led to changed environmental conditions, especially enhanced oxygen (O2) or declining hydrogen sulfide (H2S) concentrations, thus, affecting the conditions for the production of N2O. Pattern of N2O profiles and correlations with parameters like oxygen and nitrate differed between the basins. The dominant production pathway seems to be nitrification rather than denitrification. No indications for advection of N2O by North Sea Water were found. A rough budget revealed a significant surplus of in situ produced N2O after the inflow. However, due to the permanent halocline, it can be assumed that the formed N2O does not reach the atmosphere. Hydrographic aspects therefore are decisive factors determining the final release of produced N2O to the atmosphere.

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Bioavailability and remineralization of sediment-derived dissolved organic carbon from the Baltic Sea depositional area

10.5194/egusphere-egu21-7533 ◽

2021 ◽

Author(s):

Monika Lengier

Keyword(s):

Organic Carbon ◽

Dissolved Organic Carbon ◽

Baltic Sea ◽

Water Column ◽

Pore Water ◽

Half Life ◽

Bottom Water ◽

The Baltic Sea ◽

Doc Concentration ◽

The Baltic

Sediment pore waters in the depositional areas of the Baltic Sea are enriched with the dissolved organic carbon (DOC), which results in a diffusive flux of DOC to the water column. It was found that up to 30% of OM deposited in the sediments returns to the water column and may alter processes occurring there e.g. increase the oxygen demand in the bottom waters. Still little is known about the bioavailability of sediment-derived DOC and its remineralization dynamics. Thus, the aim of this study was to assess the bioavailability, degradation rate constant and half-life time of sediment-derived DOC.Bottom water and pore water, collected during r/v Oceania cruise in March 2018 in the Gda&#324;sk Deep, have been mixed in a volume ratio of 4:1. To ensure oxic conditions in the experiment, the mixture was bubbled with the ambient air to reach 100% O2 saturation. Incubation of such prepared samples was conducted in 23&#177;0.1&#176;C for 126 days. At the beginning (t=0) and after 1, 2, 6, 18, 35, 73 and 126 days of the incubation the individual samples were analyzed for total dissolved organic carbon DOC. In parallel, untreated bottom water was incubated as a control, while the obtained results have been used to decouple the remineralization dynamics in the mixture.The DOC decay had an exponential character. The highest dynamics of DOC remineralization was at the beginning of the experiment and it gradually decreased over time. During the incubation period pore water DOC concentration decreased from 1408 to 850 &#181;mol l-1, which corresponds to almost 40% loss. In the control samples (bottom water) DOC concentration decreased from 304 to 260 &#181;mol l-1 i.e. by ~14%.In the experiment three different DOC fractions have been identified: labile DOC (DOCL), semi-labile DOC (DOCSL) and refractory DOC (DOCR). To quantify the DOC remineralization rate constants (k) and half-life times (t1/2) the first order kinetics was used. The total bioavailable fraction of pore water DOC (DOCL+DOCSL) amounted to 54%, while k and t1/2 were 0.0958 d-1 and 7.24 d for DOCL and 0.0082 d-1 and 84.53 d for DOCSL, respectively.This study shows that about half of sediment-derived DOC is bioavailable, which gives a new insight on the Baltic Sea carbon cycle and O&#173;2 consumption in deeper water layers.

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Nutrient regeneration and benthic fluxes in the Coastal Baltic and North Sea

10.5194/egusphere-egu2020-15863 ◽

2020 ◽

Author(s):

Kirstin Dähnke ◽

Andreas Neumann ◽

Tina Sanders

Keyword(s):

Stable Isotope ◽

Baltic Sea ◽

Water Column ◽

North Sea ◽

Surface Sediments ◽

Nutrient Regeneration ◽

The Baltic Sea ◽

The North ◽

The North Sea ◽

The Baltic

Sediments in the coastal ocean can play an important role in nutrient regeneration and in recharging the water column with dissolved inorganic nutrients. This function, however, depends on various variables, such as physical characteristics, but also on biological traits like fauna composition and activity. To unravel and quantify these effects, we investigated nutrient fluxes and nitrate stable isotope composition in water samples along a North Sea &#8211; Skagerrak &#8211; Baltic Sea gradient during the Maria S. Merian cruise MSM 50 in January 2016.Especially in the North Sea and the Skagerrak region, d15N values of nitrate were unexpectedly high, suggesting that underlying sediments with relatively enriched isotope signatures were a source of nitrate. This nitrification signal, however, resembled an autumn situation rather than the expected winter values. Parallel sediment incubations confirm that the benthic rates of oxygen consumption and nutrient turnover were indeed very similar to respective rates in autumn and that the sediment was a source of recycled nitrate. From the North Sea towards the Baltic Sea, we found, in accordance with previous studies, a depletion in nitrate stable isotope values. This is indicative of different nitrate sources in the respective basins: in the North Sea region, N of anthropogenic origin leads to high N values in surface sediments and in newly generated nitrate. Due to a higher share of nitrogen fixation, the nitrogen stable isotope signal of surface sediments in the Baltic Sea was depleted, which in turn was mirrored in lower nitrate isotope values in the water column above the sediment.Overall, the data highlight the importance of nitrate regeneration. Parallel flux measurements reveal that faunal activity shifts the nutrient balance from sequestration to regeneration. Seasonal differences enable us to unravel seasonal effects of fauna and microbiota on nutrient budgets.

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Model calculations of the effects of present and future emissions of air pollutants from shipping in the Baltic Sea and the North Sea

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-14-21943-2014 ◽

2014 ◽

Vol 14 (15) ◽

pp. 21943-21974 ◽

Cited By ~ 4

Author(s):

J. E. Jonson ◽

J. P. Jalkanen ◽

L. Johansson ◽

M. Gauss ◽

H. A. C. Denier van der Gon

Keyword(s):

Baltic Sea ◽

North Sea ◽

Years Of Life Lost ◽

Ship Emissions ◽

The Baltic Sea ◽

Model Calculations ◽

The North ◽

Before And After ◽

The North Sea ◽

The Baltic

Abstract. Land-based emissions of air pollutants in Europe have steadily decreased over the past two decades, and this decrease is expected to continue. Within the same time span emissions from shipping have increased, although recently sulphur emissions, and subsequently particle emissions, have decreased in EU ports and in the Baltic Sea and the North Sea, defined as SECAs (Sulphur Emission Control Areas). The maximum allowed sulphur content in marine fuels in EU ports is now 0.1%, as required by the European Union sulphur directive. In the SECAs the maximum fuel content of sulphur is currently 1% (the global average is about 2.4%). This will be reduced to 0.1% from 2015, following the new IMO rules (International Maritime Organisation). In order to assess the effects of ship emissions in and around the Baltic Sea and the North Sea, regional model calculations with the EMEP air pollution model have been made on a 1/4° longitude × 1/8° latitude resolution, using ship emissions in the Baltic Sea and the North Sea that are based on accurate ship positioning data. The effects on depositions and air pollution and the resulting number of years of life lost (YOLL) have been calculated by comparing model calculations with and without ship emissions in the two sea areas. The calculations have been made with emissions representative of 2009 and 2011, i.e. before and after the implementation of stricter controls on sulphur emissions from mid 2010. The calculations with present emissions show that per person, an additional 0.1–0.2 years of life lost is estimated in areas close to the major ship tracks with present emission levels. Comparisons of model calculations with emissions before and after the implementation of stricter emission control on sulphur show a general decrease in calculated particle concentration. At the same time, however, an increase in ship activity has resulted in higher emissions and subsequently air concentrations, in particular of NOx, especially in and around several major ports. Additional model calculations have been made with land based and ship emissions representative of year 2030. Following a decrease in emissions, air quality is expected to improve, and depositions to be reduced. Particles from shipping are expected to decrease as a result of emission controls in the SECAs. Further controls of NOx emissions from shipping are not decided, and calculations are presented with and without such controls.

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