Annual variability and regulation of methane and sulfate fluxes in Baltic Sea estuarine sediments

Marine methane emissions originate largely from near-shore coastal systems, but emission estimates are often not based on temporally well-resolved data or sufficient understanding of the variability of methane consumption and production processes in the underlying sediment. The objectives of our investigation were to explore the effects of seasonal temperature, changes in benthic oxygen concentration, and historical eutrophication on sediment methane concentrations and benthic fluxes at two type localities for open-water coastal versus eutrophic, estuarine sediment in the Baltic Sea. Benthic fluxes of methane and oxygen and sediment pore-water concentrations of dissolved sulfate, methane, and 35S-sulfate reduction rates were obtained over a 12-month period from April 2012 to April 2013. Benthic methane fluxes varied by factors of 5 and 12 at the offshore coastal site and the eutrophic estuarine station, respectively, ranging from 0.1 mmol m−2 d−1 in winter at an open coastal site to 2.6 mmol m−2 d−1 in late summer in the inner eutrophic estuary. Total oxygen uptake (TOU) and 35S-sulfate reduction rates (SRRs) correlated with methane fluxes showing low rates in the winter and high rates in the summer. The highest pore-water methane concentrations also varied by factors of 6 and 10 over the sampling period with the lowest values in the winter and highest values in late summer– early autumn. The highest pore-water methane concentrations were 5.7 mM a few centimeters below the sediment surface, but they never exceeded the in situ saturation concentration. Of the total sulfate reduction, 21–24 % was coupled to anaerobic methane oxidation, lowering methane concentrations below the sediment surface far below the saturation concentration. The data imply that bubble emission likely plays no or only a minor role in methane emissions in these sediments. The changes in pore-water methane concentrations over the observation period were too large to be explained by temporal changes in methane formation and methane oxidation rates due to temperature alone. Additional factors such as regional and local hydrostatic pressure changes and coastal submarine groundwater flow may also affect the vertical and lateral transport of methane.


Methane analysis
Samples for methane were collected directly through the side of taped, pre-drilled core liners and taken 138 in 2-cm intervals seconds after the core was retrieved on deck. The core sampling method used in this 139 study permits complete sampling and preservation of porewater methane within 5 minutes after the 140 core was on deck. Under these circumstances, loss of methane due to gas loss is low and methane 141 concentrations could be determined for porewaters that were far above the saturation limit at 1 142 atmosphere pressure for the salinity and temperature range of the bottom water (between 1.9 mM and 143 2.4 mM). A sediment sample of exactly 2.5 mL was taken with a 3 mL cutoff syringe. The sample was 144 transferred to a 20 mL serum vial containing 5 mL 5 M NaCl and immediately closed with a thick 145 septum and an aluminum crimp seal. The sample was shaken, left for 1 hour for gas equilibration, and 5 146 mL of brine was injected into a sample vial to force out the 5 mL gas samples out of a vial into the 147 syringe. The CH 4 measurements were carried out on a gas chromatograph (GC) with a flame ionization 148 detector (FID) (SRI 8610C) and N 2 was used as carrier gas. CH 4 standards 100 ppm and 10000 ppm 149 (Air Liquide) were used for calibration. 150 The concentration of methane (mM) in the headspace of a sample was calculated from:  solution, followed by two rinses with deionized water for 2 hours and final storage in deionized water. 163 Rhizones were connected to 10 mL disposable plastic syringes via a 3-way luer-type stop-cock and 164 inserted in 1 cm intervals through tight-fitting, pre-drilled holes in the liner of the sediment cores. The 165 first mL of pore water was discarded from the syringe. No more than 2 ml were collected from each 166 core to prevent cross-contamination of adjacent due to the porewater suction (Seeberg-Elverfeldt et al., To determine bacterial sulfate reduction rates (SRR) sediment cores were subsampled in 40-cm 171 long 28 mm-diameter cores with 1-cm spaced, silicon-sealed, pre-drilled small holes on the side for 172 injections. For the incubation, the whole-core incubation method by Jørgensen (1978) was used. 35  factor accounting for the isotope discrimination of 35 S against 32 S-sulfate, and T is the incubation time.

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The sulfate reduction rate is reported as nmol cm -3 day -1 . 35    diffusion coefficient corrected for temperature and salinity according to Boudreau (1996)

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In agreement with the sulfate concentration gradients, 35 S-sulfate reduction rates were higher at station 287 H6 than at station B1 ( Figure 4 a-h). At station B1, SRR ranged from 0.2 nmol cm -3d -1 to 63 nmol cm-288 3 d -1 , while at H6 SRR were as high as 411 nmol cm -3 d -1 . Organoclastic sulfate reduction dominated the 289 interval down to 10 cm. Depth-integrated sulfate reduction rates over the core length varied from 9.2 to 290 11.7 mmol m -2 d -1 at station H6 and 0.5 to 2.4 mmol m 2 d -1 at station B1.

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Two distinct sulfate reduction rate peaks were found at station H6, one at the surface and a second peak  Rates of total oxygen uptake are summarized in Table 2 and shown for comparison in Figure 5. Total February to -2.7 mmol m -2 d -1 in August at station H6 (Table 2). These rates are significantly lower 317 than the radiotracer rates and indicate that sulfate is reoxidized below the sediment surface by reaction 318 with reactive iron (Thang et al., 2013). Methane fluxes determined by whole-core incubation were 319 consistently higher than the fluxes determined from the concentration profiles of dissolved methane at 320 station H6, whereas the two methods gave similar results at Station B1 ( Table 2). The seasonal 321 variability in fluxes at the two stations was similar for the two measuring methods (Table 2). Whole-322 core methane fluxes ranged from 0.3 mmol m -2 d -1 (February) to 19.9 mmol m -2 d -1 (August) at station 323 H6, and from 0.1 (February and April) to 1.2 mmol m -2 d -1 (August) at station B1 ( Figure 5, Table 2).

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The very high value measured in August 2012 at Station H6 is likely due to ebullition during the  has been interpreted as resuspended material (Blomqvist and Larsson, 1994).

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A second effect to be considered is that stations B1 and H6 are located in bathymetric depressions. H6 364 is in the center of a sub-basin separated from the outer Himmerfjärd by a sill (Fig. 1). Likewise, Station 365 B1 is located in a small depression at the head of a submarine channel that opens to the Baltic Sea. whereas the more open station B1 is affected by upwelling of oxygen-rich waters and comparatively 384 less burial of organic material (Table 1). results can be tested with our Himmerfjärden data.

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Sulfate reduction rates, particularly at H6, demonstrate how strongly bottom-water oxygen controls 408 organic matter mineralization. In the spring, summer, and fall sulfate reduction was at its maximum in 409 the first two centimeters of the sediments (Fig 3 e, f, g). In February, reduced organic carbon input and 410 higher oxygen concentrations resulted in lower sulfate reduction rates and a shift of the maximum rates 411 to greater depths in the sediments (Figure 3 h). Since other terminal carbon-oxidizing processes (e.g.   (Table 2). One possible 443 explanation for this phenomenon is therefore that rates of sulfate reduction-coupled anaerobic methane 444 oxidation, except for the winter months, were low compared to the total sulfate reduction rate. An