An intermediary scale setup to measure O2 fractionation factors of aquatic biosphere and application to the interpretation of the δ18O of O2 records found in deep ice cores.

Earth atmospheric O₂ 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, and in particular the isotopic composition of O₂ (δ¹⁸O and δ ¹⁷O). 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 uncertainties in the applicability of the previous determinations of the O₂ fractionation for the interpretation of δ¹⁸O and δ¹⁷O of atmospheric O₂.

In order to come up with coherent estimates of oxygen biological fractionation coefficients applicable to the scale of plants or ecosystems, we developed closed biological chambers as a biosphere replica, with controlled environment parameters, and measured the dynamics of O₂ concentration and of its isotopic composition.

Our set-up is based on round-bottom stainless steel tube of 10 cm in diameter and 88 cm in height to simulate a water column, on top of which we place a structure equipped with sensors (temperature, CO₂ concentration, O₂ elemental and isotopic measurements) to obtain a closed system. The multiplexing system that we developed can allow to use 6 tubes simultaneously to run replicate studies in parallel with the same environmental conditions.

We present here 3 measurement series, lasting between 2 and 9 months, run with the freshwater species, chlorella vulgaris. These measurement series permit to optimize the use of our newly developed system for aquatic closed biological chambers. We also determined the isotopic discrimination associated with ¹⁸O/¹⁶O of O₂ during respiration as -30 permil which is higher than most of the previously published values. We will also compare these results with new values measured with our setup for oceanic species (the diatoms Phaeodactylum). Finally, we will use the newly determined fractionation coefficients to improve interpretation of the δ¹⁸O of O₂ record in air bubbles from ice cores