de beer structure of vents · ph ph decrease local (
TRANSCRIPT
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Structure of ventsusing high resolution studies
Dirk de Beer, Anna Lichtschlag, Anita Flohr, Kate Peel, Dirk Koopmans, Moritz Holtappel
MPI-NOCS-AWI
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Background profiles
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Profiles in vent
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vent
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T transectsT-gradients local(<1 m from vent)
Not proportional to CO2 releaseand variable
No T in bottomwater
distance from vent (m)
0 5 10 15 20
T (C
)
7.70
7.75
7.80
7.85
7.90
7.95
8.00
dT/d
x (C
m-1
)
0
2
4
6
distance from vent (m)
0 5 10 15 20
T (C
)
7.70
7.75
7.80
7.85
7.90
7.95
8.00
dT/d
x (C
m-1
)0
2
4
6
50L/min. N-S 50L/min. E-W
distance from vent (m)
0 5 10 15 20
T (C
)
7.70
7.75
7.80
7.85
7.90
7.95
8.00
dT/d
x (C
m-1
)
0
2
4
6
distance from vent (m)
0 5 10 15 20
T (C
)
7.70
7.75
7.80
7.85
7.90
7.95
8.00
dT/d
x (C
m-1
)
0
2
4
6
distance from vent (m)
0 5 10 15 20
T (C
)
7.70
7.75
7.80
7.85
7.90
7.95
8.00
dT/d
x (C
m-1
)
0
2
4
6
bottomwaterdT/dx
distance from vent (m)
0 5 10 15 20
T (C
)
7.70
7.75
7.80
7.85
7.90
7.95
8.00
dT/d
x (C
m-1
)
0
2
4
6
0L/min 2L/min
5L/min 30L/min
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Heat generation
Loggers from Moritz Holtappel, 30 cm depth
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pH
pH decrease local(<1 m from vent)
Variable & not proportional
Local effect in bottom water
distance from vent
0 5 10 15 20
pH
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
50L/min. N-S
distance from vent
0 5 10 15 20
pH
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
50L/min. E-W
distance from vent
0 5 10 15 20
pH
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
distance from vent
0 5 10 15 20
pH
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
30L/min5L/min
distance from vent
0 5 10 15 20
pH
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
bottom watersediment
distance from vent
0 5 10 15 20
pH
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
0L/min 2L/min
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Lab experiment
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At day 4
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distance from center (m)
-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15
pH
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
7.4
Portrait of a vent
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TA values in situ
TA (mol m-3)
0 10 20 30 40 50
dept
h (m
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
TA (mol m-3)
0 10 20 30 40 50
0.00
0.05
0.10
0.15
0.20
0.25
0.30
A B
Calcite dissolution (+silicate???)Top 10 cm ventilated by escaping bubbles(Michael Stöhr: bubble out, water in)
Kate Peel, Anita Flohr
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Vent=cylinder in infinite medium
CO2->H2CO3->HCO3-+H+
Svensen, H. etal 2006 Journal of the Geological Society 163, 671-682.
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Cylindrical source: effects are local
J=k(Cb-Ci)k=D/!ℎ k=mass transfer coefficient (m s-1), δh=effective boundary layer
#ℎ = 0.3 + 0.62+,-.#/
-0
1 + (0.4#/ ).0
-5(1 + ( +,
282000)78)57
= 9:;
In absence of flow Re=0 thus: Sh = 0.3.
As #ℎ = <=> and 9 = >
?@δB = =
CB = 3:
bubble~2 cm, δB~6 cmdiameter plume ~14 cm
Churchill SW, Bernstein M. 1977. A Correlating Equation for Forced Convection From Gases and Liquids to a Circular Cylinder in Crossflow. Journal of Heat Transfer 99:300-306.
In steady state and in absence of reactions: Effective boundary layer is finite and about 3x the diameterδh independant from D, both for mass and heat
δh
Cb
Ci
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Mass loss (DIC)
δh
D? k? mass transfer mechanism?
Pulsating channel….
Carbonate equilibrium?
Kinetics of calcite dissolution?Kinetics of silicate weathering?
In sediment:>Lateral DIC diffusion
>Reactions with sediment
Above sediment:
>Plume above measuring horizon>CO2(l) (CO2→H2CO3→HCO3
-+H+ slow)
CO2 (g)→DIC
EDDY/pH sensors/DIC sensors
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75%?
• Poster Dirk Koopmans:Detected mass flow of DIC
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Heat generation1. CO2 (g) -> CO2 (aq) ΔH -20000 J mol-12. CaCO3 + CO2 (aq) + 2H2O-> Ca2+ + 2HCO3
-+ H2O ΔH -10900 J mol-13. MeSiO3 + 2CO2 (aq) + 4H2O -> MeSiO3 + 2HCO3
- + 2H3O+ ->Me2+ + H4SiO4 + 2HCO3
-+ H2O ΔH -73000 J mol-1
Heat flux: 3 J m-2 s-1
(from thermal conductivity and T gradient, underestimation!)
Assume cylinder of 3m long, 2 cm bubble channel plus 6 cm δh: 0.13 J s-1
This heat is generated by:CO2 reactions 1+2: 0.00013 mol s-1
CO2 reactions 1+3: 0.00004 mol s-1
2 L CO2 min: 0.0015 mol s-1
Stumm W, Morgan JJ. 2012. Aquatic Chemistry: Chemical Equilibriaand Rates in Natural Waters, 3 ed. Wiley.
δh
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Conclusions• Vent channels are narrow and have only local effectsØ MBL 3x diameter, if d=2 cm MBL 6 cm total diameter
influenced sediment 2+2x6=14 cm
• CO2 input generates DIC and heat• Heat and DIC partially dissipated laterally in sediment
• Heat needs inclusion in vent modeling (kinetics and viscosity)
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TA values in situ
TA (mol m-3)
0 10 20 30 40 50
dept
h (m
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
TA (mol m-3)
0 10 20 30 40 50
0.00
0.05
0.10
0.15
0.20
0.25
0.30
A B
Very variable, top 10 cm ventilatedby escaping bubbles(Michael Stöhr: bubble out, water in)Kate Peel, Anita Flohr
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Stöhr et al, 2004
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mass heat
alk 50DIC in 10000J= 0.000166667 mol/m2s 3 J/m2s
A= 0.4396 0.4396 L/minl= 3 3 2 0.001488 mol/sArea cyl 1.3188 1.3188
total flow 0.0002198 mol/s 3.9564 J/sCalcite 0.000128039 mol/sSilicate 4.25419E-05 mol/s
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Advection cell in vent
Sh ↑ thus δ"↓-> more local
Haeckel M, Boudreau BP, Wallmann K. 2007. Bubble-induced porewater mixing: A 3-D model for deep porewater irrigation. Geochimica Et Cosmochimica Acta 71:5135-5154
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Ecology and geochemistry
Sulfate reduction lower(=35% total mineralization)
No effect O2 uptake
pH decrease only local
No animals deterredSRR (mol m-3 s-1)
0.0 5.0e-8 1.0e-7 1.5e-7 2.0e-7 2.5e-7
dept
h (m
)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
ventbackground