Reconsidering the change in global biosphere productivity between the last glacial maximum and present day from the triple oxygen isotopic composition of air trapped in ice cores

We present a global model to infer past biosphere productivity using the record of triple isotopic composition of atmospheric oxygen. Our model incorporates recent determinations of the mass-dependent relationships between δ¹⁷O and δ¹⁸O associated with leaf transpiration and various O₂ uptake processes. It also considers the spatial and seasonal variations of vegetation distribution, climatic conditions, and isotopic composition of meteoric water. On the basis of this model, we provide global estimates for the Last Glacial Maximum (LGM) and the present of (1) the triple isotopic composition of leaf water, (2) isotopic fractionation factors for terrestrial dark respiration in soils and in leaves as well as total terrestrial respiration, (3) relationships between δ¹⁷O and δ¹⁸O associated with terrestrial biological steady state, and (4) ¹⁷O anomalies issued from both the terrestrial and oceanic biospheres. Using these data and the vegetation distribution simulated by the ORCHIDEE model, we estimated that the rate of global biological productivity during the LGM was 60-75% of the present rate. Our value for the LGM is at the lower end of previous estimates and suggests that the rise in biosphere productivity since the last glacial is larger than previously thought.