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Content archived on 2024-04-30

Carbon dioxide uptake by the southern ocean


Variations of atmospheric CO2 are affecting past, present and future of the global climate. The biological production of the oceans by photosynthetic fixation of dissolved CO2, as well as the related air/sea exchanges of CO2, are forcings of the abundance of CO2 in the atmosphere. The Southern Ocean plays a major role in the redistribution of CO2 between atmospheric and oceanic reservoirs. The Southern Ocean i unusual as it exhibits plenty essential nutrients nitrate, phosphate and silicate in surface water but somehow the photosynthetic CO2 fixation and plant biomass is low. It is the largest High Nutrients Low Chlorophyll (HNLC) region of the global ocean. Several independent models of the carbon cycle suggest that whatever limits the uptake of the classical nutrients in the Southern Ocean has a an over-riding influence on atmospheric carbon dioxide. From recent joint studies of several CARUSO partners it is now becoming apparent that inadequate supply of the essential trace metal iron in combination with poor light conditions preventing large diatoms to bloom and sequester CO2 into the deep sea.

Thus the overall objective of CARUSO may be formulated in one hypothesis to be tested:

"The carbon dioxide uptake by the Southern Ocean is being dominated by synergistics of light and iron regulating the photosynthetic CO2 fixation of large diatoms and carbon export into deeper waters."

For unraveling the physical forcings (light, wind, mixing) and chemical and biological processes which together control the CO2 budget in the surface ocean a carefully designed combination of four lines of observation approach and thei integration by modeling is proposed:

1) To assess on a large scale the role of the biological pump versus physical processes (upwelling, advection) with the use of transient tracers (CFC, 3He/4He, Tritium) and the purpose to obtain and understand air-sea fluxes and deep mixing and transport of CO2.

2) To unravel and understand the physiology of some key Antarctic bloom-forming diatoms (e.g. Corethron criophilum, Fragilariopsis kerguelensis, Chaetoceros) with focus on the iron-light synergistics and the carbon metabolism.

3) To assess the sources of supply of the limiting trace nutrient iron to Antarctic surface waters with the us of natural isotopic tracer techniques (228Ra and '43Nd/'44Nd).

4) To assess the effect of an artificial in situ iron fertilization on the Antarctic ecosystem and on carbon fluxes, with the use of chemical labeling techniques, in the context of a larger survey of rate and state variables.

5) To integrate the results of our, and previous, investigations in an existing mechanistic ecosystem and elemental budget (C, N, Fe, Si) model for obtaining and understanding the climatic sensitivity of above processes.
The findings of CARUSO will greatly improve our knowledge of the regulation of atmospheric CO2 and climate in present and past. This is necessary towards better reliability of predictive scenarios of climatic change, with the aim to advice policy makers within the EU and worldwide on the conceivable risks of continuing anthropogenic CO2 emissions.

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Landsdiep 4

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Participants (6)