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

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Abstract
We examine the processes responsible for the distribution of δ13C 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 δ13C distribution. Extremely slow δ13C air‐sea equilibration does not permit the surface ocean to come into equilibrium with the atmosphere and δ13C 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 δ13C 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.