Roles of biology and gas exchange in determining the δ¹³C distribution in the ocean and the preindustrial gradient in atmospheric δ¹³C

We examine the processes responsible for the distribution of δ 13 C in a global ocean model. The dominant sources of gradients are biological processes and the temperature effect on isotopic fractionation. However, in a model without biology developed to examine the temperature effect of isotopic fractionation in isolation, we find an almost uniform δ 13 C distribution. Extremely slow δ 13 C air‐sea equilibration does not permit the surface ocean to come into equilibrium with the atmosphere and δ 13 C in the ocean thus becomes well mixed. However biological effects, which are interior to the ocean, are strongly expressed and minimally effected by air‐sea exchange. Biological fractionation thus dominates the oceanic δ 13 C distribution. An important feature of the model is an extremely large northward transport of isotopic anomaly. The transfer from the ocean to the Northern Hemisphere atmosphere of 120 Pg C ‰ is equivalent in magnitude to the signal that would be generated by a net terrestrial biospheric uptake of ≈ 5 Pg C yr −1 from the Northern Hemisphere atmosphere, or an ≈ l‐2‰ disequilibrium between terrestrial respiration and photosynthesis. Improved ocean model simulations and observational analysis are required to test for the possible existence of such a large oceanic transport of isotopic anomaly.