Southern Ocean controls of the vertical marine .sup.13C gradient - a modelling study

[delta].sup.13 C, the standardised .sup.13 C / .sup.12 C ratio expressed in per mille, is a widely used ocean tracer to study changes in ocean circulation, water mass ventilation, atmospheric pCO.sub.2, and the biological carbon pump on timescales ranging from decades to tens of millions of years. [delta].sup.13 C data derived from ocean sediment core analysis provide information on [delta].sup.13 C of dissolved inorganic carbon and the vertical [delta].sup.13 C gradient (i.e. Δ[delta].sup.13 C) in past oceans. In order to correctly interpret [delta].sup.13 C and Δ[delta].sup.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 Δ[delta].sup.13 C in all ocean basins is sensitive to changes in the biogeochemical state of the Southern Ocean. We conduct a set of idealised sensitivity experiments with the ocean biogeochemistry general circulation model HAMOCC2s to explore the effect of biogeochemical state changes of the Southern and Global Ocean on atmospheric [delta].sup.13 C, pCO.sub.2, and marine [delta].sup.13 C and Δ[delta].sup.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. Our experiments show that global mean Δ[delta].sup.13 C varies by up to about ±0.35 0/00 around the pre-industrial model reference (1.2 0/00) in response to biogeochemical change. The amplitude of this sensitivity can be larger at smaller scales, as seen from a maximum sensitivity of about -0.6 0/00 on ocean basin scale. The ocean's oldest water (North Pacific) responds most to biological changes, the young deep water (North Atlantic) responds strongly to air-sea gas exchange changes, and the vertically well-mixed water (SO) has a low or even reversed Δ[delta].sup.13 C sensitivity compared to the other basins. This local Δ[delta].sup.13 C sensitivity depends on the local thermodynamic disequilibrium and the Δ[delta].sup.13 C sensitivity to local POC export production changes. The direction of both glacial (intensification of Δ[delta].sup.13 C) and interglacial (weakening of Δ[delta].sup.13 C) Δ[delta].sup.13 C change matches the direction of the sensitivity of biogeochemical processes associated with these periods. This supports the idea that biog