Structure of ventsusing high resolution studies

Dirk de Beer, Anna Lichtschlag, Anita Flohr, Kate Peel, Dirk Koopmans, Moritz Holtappel

MPI-NOCS-AWI

Background profiles

Profiles in vent

vent

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

Heat generation

Loggers from Moritz Holtappel, 30 cm depth

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

Lab experiment

At day 4

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

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

Vent=cylinder in infinite medium

CO2->H2CO3->HCO3-+H+

Svensen, H. etal 2006 Journal of the Geological Society 163, 671-682.

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

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

75%?

• Poster Dirk Koopmans:Detected mass flow of DIC

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

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)

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

Stöhr et al, 2004

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

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

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