Simulating the seasonal cycle of 13C

We investigate the seasonal cycle of δ¹³CO₂ using the Earth system model of intermediate complexity Bern3D-LPX. Using a model of atmospheric transport (TM3), the spatial fields of simulated ¹³CO₂ and CO₂ exchange are translated to local δ¹³CO₂ anomalies, which are then compared to atmospheric measurements. We discuss the ability of the model to accurately simulate the atmospheric seasonal δ¹³CO₂ cycle_, which could prove to be a valuable novel observational constraint. The coupled simulation allows us to distinguish the relative importance of the biosphere and ocean in determining the seasonal cycle of δ¹³CO₂ at different measurement sites across the world.

The amplitude of the seasonal cycle of δ¹³CO₂ is of particular importance to quantify land biosphere processes. The decreasing δ¹³CO₂ of the atmosphere during the last decades (Suess effect) leads to a divergence of the δ¹³C signature in assimilation and heterotrophic respiration, because of the long lifetime of soil pools. This is expected to lead to a high sensitivity of the seasonal amplitude to the amount of soil respiration. The effect of changes in soil turnover times on the simulated seasonal cycle is explored with factorial simulations of the Dynamic Global Vegetation Model LPX-Bern.