Inorganic carbon dynamics and air-ice-sea CO2 fluxes in the open and coastal waters of the Southern Ocean

Despite the fact that the Southern Ocean (S.O.) is a high nutrients-low chlorophyll area (HNLC), it acts as a significant sink for atmospheric CO2. We addressed the temporal and spatial variations of the frontal system of the Indian sector of the S.O. using remote sensing measurements of sea surface...

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Bibliographic Details
Main Author: Delille, Bruno
Other Authors: BELSPO - Politique scientifique fédérale sponsor, Freshwater and OCeanic science Unit of reSearch - FOCUS research center, Borges, Alberto superviser, Ronday, François superviser, Bouquegneau, Jean-Marie president of the jury, Metzl, Nicolas member of the jury, Dehairs, Frank member of the jury, Tison, Jean-Louis member of the jury, Lancelot, Chistiane member of the jury
Format: Doctoral or Postdoctoral Thesis
Language:English
Published: Université de Liège, Liège, Belgique 2006
Subjects:
Online Access:https://orbi.uliege.be/handle/2268/252964
Description
Summary:Despite the fact that the Southern Ocean (S.O.) is a high nutrients-low chlorophyll area (HNLC), it acts as a significant sink for atmospheric CO2. We addressed the temporal and spatial variations of the frontal system of the Indian sector of the S.O. using remote sensing measurements of sea surface temperature (SST) and we present the first synoptic partitioning of the main physical-biogeochemical provinces. In the Crozet basin, if the signature of the fronts is well marked in the mesoscale distribution of the partial pressure of CO2 (pCO2), this latter hardly reflects the chlorophyll a (Chl a) distribution during the summer post-bloom period. Scaling in situ pCO2 measurements using remote sensing measurements of SST and Chl a, we assessed spring and summer air-sea flux of CO2 per physicalbiogeochemical province. Spring and summer air-sea CO2 fluxes in the Indian sector of the S.O. ranges from -0.048 PgC, to -0.057 PgC and -0.04 PgC in the North Subantarctic, South Subantarctic and Polar Frontal zones, respectively. A further collaborative effort was carried out applying a similar approach to the western Pacific sector of the S.O. Integrating CO2 fluxes over the year shows that this area acts as a sink for atmospheric CO2 of 0.08 PgC yr-1. Both studies provide lower estimates than the Takahashi et al. (Takahashi et al., 2002; Takahashi, 2003) climatology but corroborate (Metzl et al., 1999; Takahashi et al., 2002) the conclusions of inverse models, indicating that this climatology overestimates the CO2 sink in the S.O. (Gurney et al., 2004; Jacobson et al., 2005). We present a three years survey of pCO2 in Subantarctic coastal waters surrounding the Kerguelen Archipelago, with a particular attention on the role of Macrocystis giant kelp beds. Primary production of Macrocystis lasts from early spring to late autumn and is tightly linked to solar irradiance. Maximum net kelp community production can be as high as 15 gC m-2 d-1 at the solar irradiance climax. Such production strongly affects pCO2 within kelp bed. Coastal waters of the archipelago experience earlier and more intense phytoplanktonic blooms than offshore waters, which markedly affect pCO2. However, over the year near-shore waters of the archipelago act as a source of CO2 of 0.32TgC yr-1. The role of sea ice cover in the budgets of exchanges of CO2 between the S.O. and the atmosphere has been neglected, since it was assumed as an impermeable and inert cover that prohibited air-sea fluxes of gases. We report the first direct measurements of pCO2 within first year pack ice, multi-year pack ice and land fast sea ice, and corresponding CO2 fluxes at the air-sea ice interface. Internal spring and summer sea ice specific processes (dilution with ice crystals, dissolution of carbonate minerals and primary production), drive drastic decreases of pCO2 and lead to marked undersaturation of CO2 with respect to the atmosphere. Despite its thinness, the Antarctic sea ice cover thus appears to sustain a significant uptake of atmospheric CO2. We scaled measurements CO2 fluxes over bare sea ice using remote sensing measurement of sea ice surface temperature: in spring and summer, the Antarctic sea ice cover acts as a sink of atmospheric CO2 ranging from 0.015 PgC to 0.024 PgC which represents 6% to 9% of the annual uptake of the S.O. south of 50°S (0.27 PgC yr-1). However, we surmise that the present evaluation of the sea ice CO2 sink is an underestimate, since it does not account for the uptake of CO2 by biologically active sea ice surface communities. Eddy-covariance measurements of air-sea ice CO2 fluxes over slush - a mixture of melting snow, ice and flooding seawater covering sea ice - which hosts abundant surface communities showed 4-fold higher fluxes than over bare sea ice. On the whole, sea ice represents an additional significant CO2 sink that should be taken into account when budgeting exchanges of CO2 fluxes between the S.O. and the atmosphere.