Southern Ocean controls of the vertical marine δ 13 C gradient – a modelling study

The standardized 13 C isotope, δ 13 C, is a widely used ocean tracer to study changes in ocean circulation, water mass ventilation, atmospheric p CO 2 and the biological carbon pump on timescales ranging from decades to 10s of millions of years. δ 13 C data derived from ocean sediment core analysis provide information on δ 13 C of dissolved inorganic carbon and the vertical δ 13 C gradient (i.e., Δδ 13 C) in past oceans. In order to correctly interpret δ 13 C and Δδ 13 C variations, a good understanding is needed of the influence from ocean circulation, air-sea gas exchange and biological productivity on these variations. The Southern Ocean is a key region for these processes, and we show here that global mean Δδ 13 C is sensitive to changes in the biogeochemical state of the Southern Ocean. We conduct four idealised sensitivity experiments with the ocean biogeochemistry general circulation model HAMOCC2s to explore the effect of biogeochemical state changes of the (Southern) Oceans on atmospheric δ 13 C, p CO 2 , and marine δ 13 C and Δδ 13 C. The experiments cover changes in air-sea gas exchange rates, particulate organic carbon sinking rates, sea ice cover, and nutrient uptake efficiency – in an unchanged ocean circulation field. We conclude that the maximum variation of mean marine Δδ 13 C in response to (bio)geochemical change is ~ 0.5 ‰, which is about half of the reconstructed variation in Δδ 13 C over glacial-interglacial timescales. Locally, Δδ 13 C variations can surpass or even mirror the mean effects on Δδ 13 C due to the spatial variation in the sensitivity of δ 13 C to biogeochemical change. The (bio)geochemical environment of a sediment core thus needs to be well constrained in order to be able to interpret reconstructed Δδ 13 C variations in such a core. The sensitivity of Δδ13C varies spatially depending on the contribution of air-sea gas exchange versus biological export productivity to the local δ 13 C signature. Interestingly, the direction of both glacial (intensification of Δδ 13 C) and interglacial (weakening of Δδ 13 C) Δδ 13 C change matches biogeochemical processes associated with these periods. This supports the idea that biogeochemistry likely explains part of the reconstructed variations in Δδ 13 C, and not only ocean circulation.