1. Numerical modeling of biogeochemical processes in gas hydrate bearing
sediments at Hydrate Ridge
Roger Luff and Klaus Wallmann
GEOMAR Research Center for Marine Geosciences
Kiel, Germany

2. Determination of: Introduction/Goals
main biogeochemical processes sediments
above gas hydrate layers
velocity of advective fluid transport through the
sediment
response of the system to changes in pore
water flow velocities
benthic fluxes of dissolved species between
sediment and bottom water

3. Cascadia Margin
TECFLUX SO (SHR) July 1999

4. (bottom boundary layer)
Model set up C. CANDI
Water column
SO42-
CO2
.....
Sulfate
Methane
CH4 OM
790 m
0.1-
1 cm
Bacterial mat
Sediment
(simulation area)
15 cm
fluid flow
Gas hydrate
(bottom boundary layer)
Transport: 16 solute, 6 solid species
advection, diffusion, bioturbation, bioirrigation
Reactions:
6 prim. redox reactions, 17 sek. redox reactions,
4 precipitation and dissolution reactions

5. Results
STEADY STATE
SIMULATIONEN

6. Results: concentrations
0 cm a-1
10 cm a-1
70 cm a-1
TVM 55/2
Pore water flux is lower than the extreme value of up to 1065 m a-1 Linke et al. (1994) for Hydrate Ridge, but falls into the range of fluxes ( cm a-1) measured by Tryon and Brown (2001) at bacterial mat sites situated on the northern and southern summits of Hydrate Ridge.
(kCH4SO4 = 100 cm3 mmol-1 a-1) falls into the range of values found in for other sedimentary environments ( cm3 mmol-1 a-1, Iversen and Jørgensen, 1985; Van Cappellen and Wang, 1996)
ARAPPT > CALPPT: Die Bakterien die Methanoxidation machen scheinen die Fällung von Aragonit zu katalysieren! Da die gemessenen Daten (Mg/Ca Verhältnis, große SO4 und kleine PO4 Konz) eher für calcite Fällung sprechen
10 mmol-1 cm–3 a-1
Kinetic const. calcite prec.
100 mmol-1 cm–3 a-1
Kinetic const. aragonite prec.
100 cm3 mmol-1 a-1
Kinetic const. CH4 oxidation
10 cm a-1
Prescribed advection velocity

7. Relative Ratentiefenprofile: Results: rates
SO42- reduction
CH4 oxidation
aragonite prec.
calcite prec.
aragonite diss.
calcite prec.
prim sek diss./prec.

8. Results: carbon budget
Source [µmol cm-2 a-1]
Sink [µmol cm-2 a-1]
Water column
Carbon turnover: 1043 [µmol cm-2 a-1]
POM 1
CO32- 86
HCO3- 2
CO2
CH4
873 81
210
CO32-
HCO3-
CO2
CH4
POM
Aut. CaCO3
120 22 1 48
843 9
TCO2
1030
Bacterial mat
Sediment
Gas hydrates

9. Results
NON-STEADY STATE
SIMULATIONEN

10. Results: fluxes Pore water fluxes, (Tryon et al., 2001)
NHR/SHR (Suess et al., 1999) Benthic chamber

11. Conclusions
Methane flux from below is the main source of
carbon at cold vent sites (83%).
Methane oxidation using sulfate is the dominate
process in this environment (84%).
Methane is almost completely oxidized by anaerobic
micro-organisms using dissolved sulfate as electron
acceptor at low fluid flow rates.
Methane charged fluids have a significant influence
on the biogeochemical processes in the sediment
column but only a minor influence on the total
methane flux into the ocean (~5.000 :
µmol cm-2 a-1).

12. Thank you!
Questions?