- The impact of declining oxygen conditions on pyrite accumulation in shelf sediments (Baltic Sea).
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Long periods of stagnation coincided with progressive eutrophication of the Baltic Sea, the beginning of which dates back to around Elmgren It is difficult to indicate the extent to which anthropogenic pressure has been responsible for these changes because they may also have been caused by the natural evolution of the Baltic Sea.
The pyrite formation in marine environment depends on the concentration of sulfide, the availability and reactivity of iron minerals, and the contact time of both components Canfield et al. For this reason, the declining oxygen conditions should be accompanied by a change in the pyrite accumulation in sediments.
In order to determine the nature of these changes, the following parameters: pyrite, acid volatile sulfides AVS , reactive iron Fe R and particulate organic carbon POC , were analyzed in dated sediments cores of approx. Finally, we have drawn conclusions on the general model of changes in pyrite content in the areas with variable oxygen conditions in the near-bottom water.
From each station, six sediment cores of approx. Cores were sliced into sections and preserved immediately after sampling. Each layer was placed in a separate polyethylene bag. Sedimentation rate and sediment age was determined in cm sediment segments, due to the fact that in deeper sediment layers the activity of Pb is below the limit of detection for the applied method.
For the purpose of that analysis, cores were sliced into 1-cm layers.
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Sediment samples from the respective layers of three cores were combined to obtain an analytical sample of suitable weight and to ensure good accuracy of the results. The first one was placed a few centimeters above the sediment—water interface to collect sample of near-bottom water. Hydrogen sulfide was analyzed with the methylene blue method after sample preservation with zinc acetate Grasshoff et al. Sulfate concentration was measured using high performance ion chromatography Methrom Professional IC. A frozen sediment sample was thawed in a N 2 -filled glove box, homogenized and divided into subsamples.
The amounts of AVS and TRS extracted from wet sediment were recalculated on dry weight using the water content in sediment. Similar precision of TRS determination was reported for example for shelf and slope sediments off the coast of Peru Suits and Arthur Leventhal and Taylor Sediment samples were freeze-dried and homogenized in agate mortar prior to analysis. About 0. The obtained extract was diluted with 0. The subsamples were mineralized in a microwave digestion system Milestone Inc.
The freeze-dried sediment samples were homogenized prior to analysis. They were placed in plastic containers of identical geometry to that used for calibration. The method of their application has been described in detail by Szmytkiewicz and Zalewska The sediment dating method based on the vertical distribution of Pb concentrations has been verified by measuring Cs activity changes along the vertical profiles of bottom sediments at all stations.
The appropriate events that increased Cs concentrations in the Baltic environment were identified in the time period covered by the dating. To determine the age of individual sediment layers, the CRS model was used. The linear equation was used to assign age to sediment layers below the depth covered by actual measurements, i.
Content of iron in the form of monosulfide Fe FeS in sediment was calculated based on the concentration of AVS, assuming that this form of sulfur consists mainly of FeS. From the original dataset, only sites with max. For each station, the oxygen values which had been measured at the maximum depth for a given depth profile were selected in the next step. Average monthly values of oxygen were computed for the further analysis.
Information on the occurrence and intensity of major Baltic inflows in years — was taken from the Digital Supplement of the book of Feistel et al. It is often applied in environmental studies to smooth lines through scatter plot or time plot Chandler and Scott We applied Statistica v. Despite only small differences max.
In the analyzed sediments, the AVS content was much lower than the pyrite content Figs. The median of FeS 2 :AVS ratio, which indicates how much Fe occurs in the form of pyrite or in the form of metastable iron sulfides Berner et al. The accumulation of AVS in sediments reflects oscillations of bottom water oxygen concentration. This greatly favors the accumulation of pyrite which is much less susceptible to oxidation Sternbeck and Sohlenius The degree of pyritization in the analyzed sediments was approximately equal to the degree of sulfidization DOS Fig.
The occurrence and intensity index of Major Baltic Inflows data from Feistel et al. The extent to which Fe in sediments is sulfidized through reaction with sulfides Eqs. This is a frequently used indicator of euxinic conditions, both for ancient and modern sediments e. Nevertheless, this indicator has some limitations.
For example, in ancient sediments the biogenic pyrite formation may proceed long after sedimentation and be unrelated to conditions in the bottom water Rickard In turn, in modern sediments, DOP can be controlled by sulfide concentration and exposure time as well as iron mineralogy Raiswell and Canfield , Iron minerals have different reactivity towards hydrogen sulfide. There are highly reactive Fe oxyhydr oxides e. Some difficulties in comparing the DOP obtained by different researchers also result from the application of various methods for Fe R determination.
All the mentioned methods elute mainly Fe oxyhydr oxides, however, the use of boiling HCl results in the removal of significant proportion of silicate-bound Fe from the sediment Raiswell et al. DOP has been calibrated by Raiswell et al. This is close to the values obtained by Canfield et al.
In the study area, the POC:TRS ranged from low values typical for euxinic environments, through normal marine to high values observed in freshwater sediments. It was from 2. Since the pyritization process depends on the concentration of sulfide, the reactivity of iron minerals and the contact time of both components Canfield et al. This may also be an effect of reoxidation of solid sulfides in the surface sediment layer during inflow events. The availability of dissolved sulfide for the reaction with iron in the study area increases strongly during long-lasting stagnation periods without any MBI.
Under conditions of abundant hydrogen sulfide, pyrite can be formed from detrital Fe minerals before sediment deposition Raiswell and Berner , and this leads to low POC:TRS ratios. As a consequence, intermediate DOP values typical for restricted marine conditions prevail.
However, it should be emphasized that besides the metastable Fe sulfides, also organic S compounds particulate and dissolved organic complexes and dissolved sulfide can be removed from the sediment with HCl Morse and Rickard In the marine environment, the factor limiting the formation of pyrite is usually the availability of organic matter or Fe oxyhydr oxides e.
Raiswell and Berner ; Sternbeck and Sohlenius Concentration of sulfates is usually high, and the rate of sulfate reduction in coastal areas is also high enough to provide sufficient amount of hydrogen sulfide Berner However, in the brackish environment, such as the Baltic Sea, it may happen that pyrite formation is limited by availability of sulfate for reduction Boesen and Postma Moreover, sulfate reduction rate within the surface sediment layer varies seasonally Aller Metabolic rate and hydrogen sulfide production decreases during the winter months when temperature and the input of organic matter to the bottom is low Hardisty et al.
The downward decrease in sulfate, resulting from dissimilatory microbiological reduction and anaerobic oxidation of methane, was observed for all sampling sites Fig. Decreasing concentration of this component was accompanied by increasing hydrogen sulfide. It was also considerably higher than the values measured in pore water of the Gotland Deep by Boesen and Postma The processes such as reaction with highly reactive iron minerals, diffusion to near bottom water and reoxidation potentially decrease sulfide concentration in the surface sediments Boesen and Postma ; Canfield et al.
As a consequence, pyrite concentration in surface sediment may be to some extent controlled by the availability of sulfide. Concentration of sulfide was probably insufficient to convert AVS to pyrite, which is consistent with two-step mechanism of formation of this mineral as proposed among others by Berner and Wilkin and Barnes For comparison, magnetite reacts with sulfide about 10 4 —10 6 and Fe-containing silicates about 10 8 more slowly Canfield et al.
The rate of sulfate reduction is greater than the rate of reaction between sulfide and iron and, as a result, hydrogen sulfide builds up in pore water Fig. Similar situation was observed for example by Canfield et al.
Sulfidic Sediments and Sedimentary Rocks, Volume 65
One way of determining which factor limits pyrite formation is to analyze relations between POC and other parameters, i. In such environment, pyrite is formed after sediment deposition and the rate of mineral formation is controlled by the sedimentation rate and the amount of organic matter Raiswell and Berner Berner ; Raiswell and Berner ; Boesen and Postma Solid and dashed lines presented on plot a are mean and range of POC:TRS ratios in normal marine sediment deposited under oxic conditions.
For the purpose of these comparisons, only layers deposited after were used. The average values of the analyzed parameters were calculated based on results obtained for particular sediment layers deposited in the same year at all examined stations. For years in which more than one inflow occurred the volume of all events were summed.
The reactions take place as long as fresh FeS surfaces continue to be exposed to weakly oxidizing conditions. Therefore, it is likely that the discussed mechanism has a significant impact on FeS 2 formation in the study area.
It should be noted, however, that before about , the inflows and FeS 2 concentrations were approximately inversely correlated Fig. Probably the proportions between oxygen concentration in bottom waters and the amount of organic matter reaching the bottom were such that the improvement of oxygen conditions after the inflows significantly limited reduction of sulfates and, consequently, the formation of iron sulfides. The importance of reactive iron as a limiting factor increased after when deterioration of oxygen conditions occurred.
The decrease in oxygen concentration around results mainly from the weakened ventilation of bottom water. In the subsequent years, a further decline in oxygen conditions resulting from eutrophication has occurred in the study area. The excessive nutrient input from land has caused an increase in sedimentation of organic material, leading to the imbalance between oxygen supply from inflows and oxygen consumption in mineralization processes.
Exhalative Origins of Iron Formations
The deterioration of oxygen conditions have lowered the availability of Fe R , which in turn became the limiting factor for pyrite formation. Temporary aerobic conditions following the inflows of highly saline surface water from the North Sea MBIs promote acid volatile sulfide oxidation in the surface layer of sediment, which in turn greatly favors the accumulation of pyrite in relation to that of metastable iron sulfides.
The improvement in oxygen conditions causes the oxidation of hydrogen sulfide, which becomes the main factor limiting pyrite formation during MBIs. The changes in oxygen conditions in bottom water are reflected in highly variable degree of pyritization and organic carbon to pyrite sulfur ratio in sediments of the study area.
The variability of pyrite, organic carbon and reactive iron content obtained for the study area indicate that, during the period from ca. After , the importance of reactive iron as a limiting factor increased. We propose that, the observed alteration of the factor limiting pyrite formation, was caused by expansion of bottom water anoxia and hypoxia due to poor ventilation i. Similar effect of prolonged stagnation periods can be expected for other Baltic deeps.