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- Title
Southern Ocean controls of the vertical marine δ13C gradient – a modelling study.
- Authors
Morée, Anne L.; Heinze, Christoph; Schwinger, Jörg
- Abstract
The standardized 13C isotope, δ13C, 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 10s 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 global mean Δδ13C 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 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. We conclude that the maximum variation of mean marine Δδ13C in response to (bio)geochemical change is ~ 0.5 ‰, which is about half of the reconstructed variation in Δδ13C over glacial-interglacial timescales. Locally, Δδ13C variations can surpass or even mirror the mean effects on Δδ13C due to the spatial variation in the sensitivity of δ13C 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 Δδ13C 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 δ13C signature. Interestingly, the direction of both glacial (intensification of Δδ13C) and interglacial (weakening of Δδ13C) Δδ13C change matches biogeochemical processes associated with these periods. This supports the idea that biogeochemistry likely explains part of the reconstructed variations in Δδ13C, and not only ocean circulation.
- Subjects
ANTARCTIC Ocean; CARBON isotopes; OCEAN circulation; MARINE sediment analysis; ATMOSPHERIC carbon dioxide
- Publication
Biogeosciences Discussions, 2018, p1
- ISSN
1810-6277
- Publication type
Article
- DOI
10.5194/bg-2018-52