Seasonal controls of methane gas solubility and
transport on anaerobic oxidation of methane in shallow water marine sediments
José M. Mogollón
1*, Andy Dale
1, Ivan L'Heureux
2, and Pierre Regnier
1,31 Faculty of GeoSciences, Utrecht University. Utrecht, The Netherlands 2 Department of Physics, University of Ottawa. Ottawa (ON), Canada
References:
Dale et al., 2008, Geology, Accepted
Martens et al., 1998, Cont. Shelf Res., v. 18, p. 1741-177 Martens et al., 1999, Amer. Journ. Sci., v. 299, p. 589-610 Mogollón et al., 2009, Amer. Jour. Sci., v 309, p. 189-220
Schlüter et al, 2000, Geochim Cosmochim Acta, v. 64 p.821–834 Treude et al, 2005, Limnol. and Oceanogr., v. 50, p. 1771-1786 Wever et al., 2006, Mar. Geo., v. 225, p. 1-4
Acknowledgements:
This project was funded by NWO Vidi Award #864.05.00 Variations in temperature at the sediment-water interface (fig. 6a) lead to
heat diffusing into the sediments. The heat capacity of the sediment
produces lag times reflected in both the temperature profiles (fig. 6B) and the monthly variations in the methane bubble depth (MBD) (fig. 6C). In the early winter, when the gas is shallowest, the propensity for CH4(g) escape increases.
Temperature
Conclusions
Controls on methane solubility
Steady-state calibration
Fig. 1: a Sketch of the geochemical processes during low (top) and
high (bottom) CH4 saturation
During pressure drops, the methane solubility will
decrease and stimulate gas formation in the porewater.
The magnitude of the gas formation and its increase with respect to the steady- state gas pool will depend on both the magnitude and the duration of the pressure drop (Fig 5). For Eckernförde Bay, tidal fluctuations and atmospheric pressure
changes are not long and sufficient enough to trigger gas escape.
Pressure effects on gas inventory
Gas effects on AOM
Increasing POC fluxes increase the SR rates, amplifying the zone of MET and thus the CH4 (g) generated and AOM intensity. If CH4 (g) is ignored (implicit - Fig.
7), integrated AOM rates reach a plateau and may be underestimated with respect to simulations that include CH4 (g) (explicit - Fig. 7). Furthermore, a
temperature drop will both decrease in the intensity of microbial activity and increase gas dissolution rates (Fig. 8). These opposing processes for AOM will shift in magnitude for sediments according to the MBD: When shallow,
temperature dominates and when deep gas dissolution dominates.
Introduction
The strong correlation
between the MBD and CH4 fluxes to the SMTZ can lead to predictions of the CH4 turnover rates (integrated AOM rates) when CH4(g) escape from the sediment is negligible. Dale et al.
(2008) developed MBD-
CH4 flux curves for various CH4 saturations based on acoustic survey profiles (a standard mapping
technique) and measured AOM rates. Our preliminary seasonal simulations show the same effect (Fig. 9).
Fig. 7. Integreted AOM vs. POC fluxes
Fig. 9. Methane fluxes as a function of the methane bubble depth
J F M
Fig. 6: A Temperature data points and fit at the SWI. B: Yearly
temperature profiles at Eckerförde Bay. C: Gas phase seasonality and comparison to acoustic data.
A
B
A M J
J A S O N D J F
M A M J
J A S O N D
Wever et al (2006)
Temperature (K)
Depth(cm)
270 272 274 276 278 280 282 284
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700
January 1st February 6th March 14th April 20th May 26th July 2nd August 7th September 13th October 19th November 25th December 31st
Time (days) Temperature(0 C)
0 100 200 300
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Schlutter et al (2003) Treude et al (2005) Curve fit
Temperature at the sediment-water interface
Time (days) Temperature(0 C)
0 100 200 300
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Schlutter et al (2000) Treude et al (2005) Curve fit
Temperature at the sediment-water interface
C
Fig. 4 Simulated and measured (Martens et al., 1999, ) geochemical
rates at Eckernförde Bay
Rates (mM/y)
Depth(cm)
0 1 2 3 4 5
0 50 100 150 200 250 300
AOM (meas) AOMTotal Sulf Red Methanogenesis
Rates (mM/y)
Depth(cm)
0 1 2 3 4 5
0 50 100 150 200 250 300
(measured)
Predicting CH4 fluxes from MBD
Metha ne flux (nmolc m-2 day-1)
Methanebubbledepth(cm)
0 100 200 300 400 500
0
50
100
150
200
C H4 s olubility = 5 mM C H4 s olubility = 7 mM C H4 s olubility = 9 mM E B1
A B1 E B2
D ec embe r
J uly
A pril
N ov embe r
J une
S epte mber
F ebruary
(MBD) (cm)
- Seasonal variations in temperature at the SWI lead to pronounced changes in solubility and, consequently the methane bubble depth.
- Pressure drops due to storms and intense wind activity may lead to ample gas formation, but at the daily time scales these are too short to allow gas escape for MBDs > 50 cm
- Seasonal AOM cycles are influenced by the dissolution of CH4 (g), which sustains the CH4 (aq) concentrations as methane is consumed in AOM.
Fig. 3: Simulated and measured
(Martens et al., 1998, Wever et al., 2006) geochemical profiles at Eckernförde Bay
Concentration (mM)
Gas Volume Fraction
Depth(cm)
0 5 10 15
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0
50 100 150 200 250 300
SO42- sured) SO42-
CH4 (aq) (meas.) CH4 (aq)
CH4 (sat) CH4 (gas)
(measured) (measured)
SMTZ
P OC flux (mol C /(m2a)) IntegratedAOMrate(molCH 4/(m2 a))
1 1.5 2 2.5 3
0.2 0.4 0.6 0.8 1 1.2
E xplicit rates Implicit rates E xplicit trend Implicit trend
y = 0.39x - 0.1772 R2= 0.9963
y = 0.2679Ln(x) + 0.1814 R2= 0.9987
Universiteit Utrecht
Pressure (bar)
3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4
274 275 276 277 278 279 280 281 282 283
Temperature (K)
5 5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8
5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2 8.4 8.6
Methane solubility concentration (mM)
Salinity = 24 Salinity = 2.4
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7
Dutation of perturbation (days)
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7
50 100 150 200 250 300 350 400 450 500
Sea level drop (cm)
50 100 150 200 250 300 350 400 450 500
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Increase in the integrated gas phase (%)
The solubility of free methane gas depends on the local pressure, temperature and
salinity. Figure 2 shows a calibrated algorithm that predicts the
methane solubility as a function of these parameters for
shallow water
conditions. These
parameters may vary accros time
A 1D, 3-phase model was built to explore the fate of CH4 (g) in Eckernförde Bay (Mogollón et al., 2009). The model was calibrated with measured
concentration and rate profiles from Martens et al (1998, 1999) (Figures 3,4) Particulate organic carbon (POC) degradation
coupled to SO42- reduction takes place in the upper parts of anoxic sediments. Once SO42-
reaches sub-mM concentrations, methanogenesis (MET) begins. When MET is high, the local CH4 (aq) may exceed the CH4 solubility leading to CH4 (g)
formation. Upward gas migration and dissolution in the overlying undersaturated sediment
enhance methane consumtion in the presence of SO42- through a process known as anaerobic
oxidation of methane (AOM). If sufficient, gas may also escape the AOM barrier and leave the
sediment. Seasonal variations in the CH4 solubility can leat to times of preferetial CH4 (g) production and CH4 (g) dissolution (Fig 1), which are crucial for determining the methane cycle.
Average bubble radius
SMTZ
CH4 CH2O
SO42-
4SMTZ
Dissolution
Formation
Average bubble radius
SMTZ
CH4 CH2O
SO42-
4SMTZ
Dissolution
Fig. 2. CH4 saturation variations for a shallow (< 30 m) marine to brackish sediment
Fig. 5. Variations in the integrated gas pool with respect to the steady-state values (Figs
2,3) due to sustained pressure drops
IntegratedAOMrate(mmolm-2 a-1 ) Integratedgaspool(mmolm-2 )
300 325 350 375 400 425 450 475 500
0 100 200 300 400 500 600 700 800 900 1000
A OM G as pool
J F M A M J J A S O N D
Fig. 8. Seasonality ofthe integrated AOM rates and the integrated gas pool
* j.mogollon@geo.uu.nl
3 Department of Earth and Environmental Sciences, Université Libre de Bruxelles. Brussels, Belgium
