A multiplexing set-up of aquatic biological chambers to study the isotopic fractionation of oxygen: application to the interpretation of the δ18O of O2 records found in deep ice cores.

Earth atmospheric dioxygen is mainly produced by biosphere photosynthesis, and biosphere respiration is also one of the main consumers of this gas. The evolution of atmospheric O₂ is thus linked to global biosphere productivity.

 

In ice cores we extract air from bubbles to study the composition of the past atmosphere. However, as O₂ concentration in air bubbles is affected by close off processes, it is difficult to reconstruct its variations in the past atmosphere from ice core analyses. In turn, the isotopic composition of O₂ (δ¹⁸O and δ ¹⁷O), is also influenced by biological processes and is less influenced by close-off processes so that this tracer should provide useful information on the past biosphere activity.

 

Quantitative interpretation of the isotopic composition of O₂ in the past relies on robust estimate of oxygen fractionation coefficients associated with the relevant biological processes: photosynthesis and respiration. In the past decades, some determinations of these biological fractionation coefficients were performed in uncontrolled large-scale environments or at the scale of the micro-organisms in conditions very different from the natural environment. There are thus inconsistencies in previous determinations of the O₂ fractionation coefficients limiting the interpretation of δ¹⁸O and δ ¹⁷O of O₂.

 

In order to come up with coherent estimates of oxygen fractionation coefficients during biological processes, we developed closed biological chambers as a biosphere replica, with controlled environment parameters (light, temperature, CO₂ concentration), which were used in combination with a newly designed optical spectrometer for continuous measurements of O₂ concentration and of its isotopic composition.

 

In this presentation, we show the design and realisation of our aquatic biological chambers as well as the associated development of the multiplexing system to be able to run parallel experiments with the same environmental conditions. Then, we show the results obtained for light and dark periods, and the corresponding fractionation coefficients calculated for photosynthesis and respiration. Finally, we use the newly determined fractionation coefficients to improve interpretation of the δ¹⁸O of O₂ record in air bubbles from ice cores.