Towards understanding global variability in ocean carbon‐13

We include a prognostic parameterization of carbon‐13 into a global ocean‐biogeochemistry model to investigate the spatiotemporal variability in ocean carbon‐13 between 1860 and 2000. Carbon‐13 was included in all 10 existing carbon pools, with dynamic fractionations occurring during photosynthesis, gas exchange and carbonate chemistry. We find that ocean distributions of δ 13 C DIC at any point in time are controlled by the interplay between biological fractionation, gas exchange, and ocean mixing. In particular, the deep ocean δ 13 C DIC is sensitive (by > 0.5‰) to the degree of ocean ventilation. On interannual timescales, although the variability in δ 13 C DIC is a first order function of the atmospheric δ 13 CO 2 and overall carbon flux, the spatial distributions are controlled by the degree to which surface waters are exposed to the atmosphere. The δ 13 C POC is highly sensitive to the species of inorganic carbon assimilated during photosynthesis (by 10 to 17‰), as well as the intrinsic growth rate and in situ [CO 2 (aq)], suggesting that phytoplankton utilize both HCO 3 − and CO 2 (aq). The relationship between Δ δ 13 C DIC and anthropogenic carbon (C ant ) varies by ±70% regionally and circulation and biotic effects can influence estimates of C ant that are based on Δ δ 13 C DIC .