The study of air-ice CO2 exchange emphasize the importance of gas bubble transport during sea ice growth

(1)

D a y o f th e e x p e r im e n t 0 2 4 6 8 1 0 1 2 1 4 1 6 p C O2 ( ma tm ) 0 2 0 0 4 0 0 6 0 0 A t m o s p h e r ic p C O2 I c e s u r f a c e p C O2 D a y o f th e e x p e r im e n t 0 2 4 6 8 1 0 1 2 1 4 1 6 A ir -i c e in te rf a c e t e m p e ra tu re ( °C ) -1 0 -9 -8 -7 -6 -5 -4 -3 -2 A ir -i c e C O2 f lu x ( m m o l m -2 d -1) -6 -2 -1 0 1 C O2 s in k C O2 s o u r c e

Gas transfer coefficient :

K = F / (pCO

_{2 ice}

- pCO

_{2 air}

) =

2,5

mol m

-2

_d

-1

_atm

-1

The study of air-ice CO

₂

exchange emphasize the importance of gas

bubble transport during sea ice growth

TAKE HOME MESSAGES:

 Sea ice exchange gas with the atmosphere

 During sea ice growth, gas are transported by diffusion and buoyancy while convection is limited

 The gas transfer coefficient for CO2 at the air- ice

interface of growing sea ice is 2,5 mol m-2 _d-1_atm-1

 Our 1D model can simulate air-ice CO2 fluxes and

point to the role of gas bubbles in sea ice

 Gas bubbles may explain a large part (~ 92 %) of the observed CO2 fluxes in the present experiment

Cooling ->

Marie Kotovitch1,2_{, Sébastien Moreau}3_{, Jiayun Zhou}1,2_{, Jean-Louis Tison}2_{, Fanny Van der Linden}1_,

Gerhard Dieckmann4_{, Karl-Ulrich Evers}5_{, David Thomas}6,7_{and Bruno Delille}1

1D MODELLING EMPHASIZES THE ROLE OF BUBBLE

Carbon dioxide (CO2) plays a crucial role in climate changes and global warming. Sea ice is

involved in CO2 exchanges between the ocean and the atmosphere. In the present study, we used a tank that allows to

control freezing of seawater in order to reproduce ice growth and decay over a 19 days experiment. The aims of this study were to: (1) determine a gas transfer coefficient for CO2 in sea ice during its growth, (2) understand the processes

responsible for CO2 transport between air and sea ice by comparing our results with a 1D biogeochemical model.

Materials and Methods: The temperature of the atmosphere above the tank water was set to -15 °C the first 14 days, then

to -1 °C until the end of the experiment. We measured continuously in situ ice temperature, underwater salinity and air-ice CO2 fluxes. Ice cores were also collected regularly to measure biogeochemical variables.

INTRODUCTION

F = air-ice CO

2

fluxes

F = air-ice CO

2

fluxes

Warming -> Cooling ->

We measured temperature and air-ice CO2 flux

in continuous with a chamber over the ice. Sea ice shifts from:

(i) a sink during the first crystals formation,

(ii) to a source during ice growth and, finally

(iii) return to a sink during ice melt.

From the difference in partial pressure of CO2

in the air and the ice, we calculated daily dC.

The evolution of dC is consistent with measured flux (F), shifting from: (i) negative,

(ii) to positive and, finally (iii) return negative.

We define dx from the observation of the total inorganic carbon (DIC) profiles. We observed a CO2

depletion in the 12 first centimetres. We assumed that it was due to a CO2

release from the ice to the air.

dC = air-ice difference in CO

2

partial pressure

dC = air-ice difference in CO

2

partial pressure

dx = depth in ice for air-ice

CO

2

exchange

dx = depth in ice for air-ice

CO

2

exchange

We included the 3 parameters determined above in the equation define by Liss and Slater (1974) and Sarmiento and Gruber (2004) and deduced a gas transfer coefficient for CO2 (K). This coefficient is applicablefor a growing permeable and artificial sea ice.

References: Moreau et al., 2015. Drivers of inorganic carbon dynamics in first-year sea ice : a model study. J. Geophys. Res. Ocean. 120, 471–495.

To mimic the observed air-ice CO2 fluxes, we used the 1D model of Moreau et

al. (2015). The inversion between outward CO2 fluxes during ice growth and

inward CO2 fluxes during ice melt depicts well the observations. However, the

model (M2015) strongly underestimates the fluxes during the cold phase if the

formation rate of gas bubbles is low. Since ice is permeable throughout the

cold phase, higher gas bubble formation rates lead to higher CO2 fluxes (i.e.

gas bubbles escape the ice surface and add to the outward CO2 flux).

1_{Unité d’Océanographie Chimique, Ulg, Belgium,} 2_{Laboratoire de}

Glaciologie, ULB, 3_{Georges Lemaître Centre for Earth and Climate}

Research, UCL, 4_{Alfred Wegener Institute Helmhotz Center for Polarand}

Marine Research, Germany, 5_{Hamburgische Schiffbau-Versuchsanstalt}

GmbH (HSVA), 6_{Marine Research Center, Helsinki, Finland,}7_{School of} Ocean Sciences, Bargor University, United Kingdom

Warming -> D IC 7 ( µ m o l k g - 1₎ 4 0 0 4 5 0 5 0 0 D ep th ( cm ) 0 5 1 0 1 5 2 0 2 5 D a y 5 D a y 8 D a y 1 4 D a y 1 5 D a y 1 6 D a y o f th e e x p e r im e n t 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 A ir -ic e C O2 fl u xe s (m m o l m -2 d -1) -1 .0 -0 .5 0 .0 0 .5 1 .0 O b s H ig h B u b C T R L L o w B u b M o r e a u 2 0 1 5