PowerPoint-Pr

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
rluff_at_geomar.de
2
Introduction/Goals
Determination of

• 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 SO143 55-2 (SHR) July 1999
4
Model set up C. CANDI
Water column
790 m
0.1- 1 cm
Bacterial mat
Sediment (simulation area)
15 cm
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
Major processes
Main rate laws applied in the model
Anaerobic methane oxidation
Carbonate (aragonite calcite) precipitation
6
Results
STEADY STATE SIMULATIONEN
7
Results concentrations
8
Results rates
Relative Ratentiefenprofile
SO42- reduction
CH4 oxidation
aragonite prec.
calcite prec.
aragonite diss. calcite prec.
prim. sek.
diss./prec.
9
Results carbon budget
Source µmol cm-2 a-1
Sink µmol cm-2 a-1
Water column
Bacterial mat
Sediment
Gas hydrates
10
Results SO42- penetration depth
Variations - depth of gas hydrate 5 cm 100 cm
in 5 cm steps - pore water velocity 0 cm a-1 to
500 cm a-1 in 1 cm a-1 steps ? 10.000 steady
state simulations
10 cm a-1
500 cm a-1
0 cm a-1
11
Results SO42- penetration depth
Measurements of the SO42- penetration depth at
bacterial mat sites at Cascadia Margin
n18
12
Results
NON-STEADY STATE SIMULATIONEN
13
Results reaction of the sediment
Pore water fluxes, (Tryon et al., 2001)
Ca concentration mmol l-1
SO42- concentration mmol l-1
14
Results fluxes
Pore water fluxes, (Tryon et al., 2001)
15
Results methane flux comparisons
CH4-fluxes from benthic chamber measurements
Position
Flux µmol cm-2 a-1
Area m2
0.26
4.300
NHR (Linke et al., 1994)
NHR/SHR (Suess et al., 1999)
4.560-13.700
0.26
CH4-fluxes from CTD-data (480-860m) (Heeschen,
2002)
Flux µmol cm-2 a-1
Area km2
Position
61
37
SHR
117.000
0.03
SHR inner box
16
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
(5.000 117.000 µmol cm-2 a-1).
17
Outlook
OUTLOOK
18
Outlook Modelling carbonate crust formation
Porosity is a function of CaCO3 concentration
DB00.01 cm2 a-1
DB00. 1 cm2 a-1
DB00.2 cm2 a-1
U10 cm a-1
U20 cm a-1
U30 cm a-1
W0.027 cm a-1
W0.05 cm a-1
W0.1 cm a-1
DB0
bioturbation coefficient
U
vent flow (pore water)
W
sedimentation velocity
19
Thank you!
Questions?
rluff_at_geomar.de