Sea Ice CO 2 Dynamics Across Seasons: Impact of Processes at the Interfaces

• Published in: Journal of geophysical research. Oceans Vol. 125; no. 6
• Main Authors: Van der Linden, F. C., Tison, J.‐L., Champenois, W., Moreau, S., Carnat, G., Kotovitch, M., Fripiat, F., Deman, F., Roukaerts, A., Dehairs, F., Wauthy, S., Lourenço, A., Vivier, F., Haskell, T., Delille, B.
• Format: Journal Article
• Language: English
• Published: 01-06-2020
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2019JC015807

Winter to summer CO
2
dynamics within landfast sea ice in McMurdo Sound (Antarctica) were investigated using bulk ice pCO
2
measurements, air‐snow‐ice CO
2
fluxes, dissolved inorganic carbon (DIC), total alkalinity (TA), and ikaite saturation state. Our results suggest depth‐dependent biotic and abiotic controls that led us to discriminate the ice column in three layers. At the surface, winter pCO
2
supersaturation drove CO
2
release to the atmosphere while spring‐summer pCO
2
undersaturation led to CO
2
uptake most of the time. CO
2
fluxes showed a diel pattern superimposed upon this seasonal pattern which was potentially assigned to either ice skin freeze‐thaw cycles or diel changes in net community production. In the ice interior, the pCO
2
decrease across the season was driven by physical processes, mainly independent of the autotrophic and heterotrophic phases. Bottom sea ice was characterized by a massive biomass build‐up counterintuitively associated with transient heterotrophic activity and nitrate plus nitrite accumulation. This inconsistency is likely related to the formation of a biofilm. This biofilm hosts both autotrophic and heterotrophic activities at the bottom of the ice during spring and may promote calcium carbonate precipitation.
Sea ice participates actively in the regional cycling of CO
2
both as a source and a sink at different times of the year depending on ice physics, ice chemistry, and ice trophic status (autotrophic vs. heterotrophic). We identified the key processes driving the CO
2
dynamics in each sea ice layer (surface, interior, and bottom) from McMurdo Sound (Antarctica) from late winter to summer. At the surface, CO
2
release from the ice to the atmosphere occurred in late winter while CO
2
uptake occurred in summer. Superimposed upon this seasonal pattern, we observed a diurnal pattern with both release and uptake occurring over 24 hr period. This diurnal pattern can be related to physical processes (nocturnal freeze‐up and diurnal melting) or biotic processes (autotrophy or heterotrophy). In the ice interior, a succession of autotrophic and heterotrophic phases took place. At the sea ice bottom, a particular assemblage of microbial cells and organic matter, called biofilm, enabled the accumulation of biomass and nitrate plus nitrite simultaneously leading to both autotrophic and heterotrophic activities. In addition, this biofilm is suggested to promote calcium carbonate precipitation.
First long‐term monitoring of both pCO
2
and CO
2
fluxes at sea ice interfaces in McMurdo Sound (Antarctica) from late winter to summer
Large biomass build‐up is strikingly associated with transient heterotrophy and nitrate plus nitrite accumulation
New conceptual approach involving biofilm formation can explain observed inconsistencies: accumulation of nitrate plus nitrite and CaCO
3