Abstract
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.
Highlights
The vertical marine δ13C gradient ( δ13C) is the surface-todeep difference in δ13C of dissolved inorganic carbon (DIC), where the standardised 13C / 12C ratio (δ13C) is expressed in per mille (Zeebe and Wolf-Gladrow, 2001): δ13C =13C/12C 13C/12C standard − 1 × 1000 ‰. (1)Here, 13C / 12Cstandard is the Pee Dee Belemnite standard (0.0112372) (Craig, 1957). 13C is slightly heavier than the 12C isotope, which causes a fractionation effect during air– sea gas exchange and photosynthesis, thereby changing δ13C and δ13C (Laws et al, 1997; Zhang et al, 1995; Mackenzie and Lerman, 2006)
This study addresses the sensitivity of modelled marine and atmospheric δ13C and δ13C to changes in biogeochemical parameters under constant ocean circulation, focusing on the contribution of the Southern Ocean (SO)
Variations in δ13C recorded in sediment records are sensitive to ocean circulation changes as shown in previous studies, but here we show that the biogeochemical state of the (Southern) Ocean can have large effects on δ13C across all ocean basins
Summary
The vertical marine δ13C gradient ( δ13C) is the surface-todeep difference in δ13C of dissolved inorganic carbon (DIC), where the standardised 13C / 12C ratio (δ13C) is expressed in per mille (Zeebe and Wolf-Gladrow, 2001): δ13C =13C/12C 13C/12C standard − 1 × 1000 ‰. (1)Here, 13C / 12Cstandard is the Pee Dee Belemnite standard (0.0112372) (Craig, 1957). 13C is slightly heavier than the 12C isotope, which causes a fractionation effect during air– sea gas exchange and photosynthesis, thereby changing δ13C and δ13C (Laws et al, 1997; Zhang et al, 1995; Mackenzie and Lerman, 2006). The vertical marine δ13C gradient ( δ13C) is the surface-todeep difference in δ13C of dissolved inorganic carbon (DIC), where the standardised 13C / 12C ratio (δ13C) is expressed in per mille (Zeebe and Wolf-Gladrow, 2001): δ13C =. 13C/12C 13C/12C standard − 1 × 1000 ‰. 13C / 12Cstandard is the Pee Dee Belemnite standard (0.0112372) (Craig, 1957). Photosynthetic fractionation increases the 13C / 12C ratio of surface ocean DIC (i.e. a δ13C increase) due to the preferred uptake of the lighter 12C into biogenic matter (which has a low δ13C). The deep sea DIC has a relatively low δ13C signature as a result of the remineralisation of low-δ13C biogenic matter at depth. Morée et al.: Southern Ocean controls of the vertical marine δ13C gradient
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