Abstract. δ13C, the standardised 13C ∕ 12C ratio
expressed in per mille, is a widely used ocean tracer to study changes in ocean
circulation, water mass ventilation, atmospheric pCO2, and
the biological carbon pump on timescales ranging from decades to tens of
millions of years. δ13C data derived from ocean sediment core
analysis provide information on δ13C of dissolved inorganic
carbon and the vertical δ13C gradient (i.e. Δδ13C) in past oceans. In order to correctly interpret
δ13C and Δδ13C 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 Δδ13C 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 δ13C,
pCO2, and marine δ13C and Δδ13C. 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 Δδ13C varies by up to about ±0.35 ‰ around the pre-industrial model reference (1.2 ‰) 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 ‰ 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
Δδ13C sensitivity compared to the other basins. This
local Δδ13C sensitivity depends on the local
thermodynamic disequilibrium and the Δδ13C sensitivity
to local POC export production changes. The direction of both glacial
(intensification of Δδ13C) and interglacial (weakening of
Δδ13C) Δδ13C change matches the
direction of the sensitivity of biogeochemical processes associated with
these periods. This supports the idea that biogeochemistry likely explains
part of the reconstructed variations in Δδ13C, in
addition to changes in ocean circulation.