Integrative Research Approach

The brief presentation of the information content of atmospheric molecular oxygen and the oxygen isotopes focused primarily on the individual merits of each tracer. However, as evident from the discussion of the combined CO, and 02 budgets, there exists considerable additional synergy if the tracer information is used in a combined, integrative way. Up to now, studies that combine the information from several of these tracers have not been attempted, in part because abundant atmospheric observations with sufficient measurement accuracy are not yet readily available. In particular, the oxygen isotope ratio measurements in atmospheric 02 are only now becoming precise enough to reveal spatial and temporal patterns in the present-day atmosphere. If they were so precise, however, the potential of an integrative approach would be substantial: The tracers discussed above provide a total of six independent constraints (concentration of C02 and 02, l80/]60 in C02 and 02, and ,70/l60 in CO, and 02), which may be used in a combined way to quantitatively deduce the six major carbon fluxes of interest including photosynthesis, respiration on land and in the sea, together with the gross air-sea gas exchange fluxes. Of course, the application of this approach requires the knowledge of:

1. the processes occurring at linkage points between the oxygen cycle and the carbon cycle, (e.g., stoichiometry between biological uptake and release of 02 and CO, on land and in the sea;

2. the fractionation processes involved at the phase transitions; and

3. the isotopic composition of the water that is imparted to 02

and CO, formed during photosynthesis or respiration.

This information must be available on the temporal and spatial scales of interest. It remains a research challenge for the next few years to develop a modeling framework into which the tracer in formation can be integrated, possibly by means of advanced data assimilation methods.

Observations of past temporal variations of the isotopic com position in ice-core O, (Bender et al, 1994; Luz et al, 1999) provide a further challenge. The tight coupling between the cycles of carbon, oxygen, and water necessitates an earth-system modeling approach in which modules representing the terrestrial and oceanic carbon cycle are coupled into a global climate model that would also include a description of the oxygen isotopes in the hy drological cycle. Such models are currently being developed. For such a model framework the oxygen isotopes in O, will provide a promising model-validation tool in the future (Hoffmann et al,

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