The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere and ocean

• Published in: Global Biogeochemical Cycles
• Main Authors: Galbraith, E.D., Kwon, E.-Y., Bianchi, D., Hain, M.P., Sarmiento, J.L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2015
• Subjects: Southern Ocean; North Atlantic Deep Water; North Atlantic
• Online Access: https://eprints.soton.ac.uk/374713/; https://eprints.soton.ac.uk/374713/1/Galbraith%2520et%2520al%25202015%2520GBC%2520CO2%2520carbon%2520isotopes.pdf; https://eprints.soton.ac.uk/374713/2/gbc20251.pdf

It is well known that the equilibration timescale for the isotopic ratios 13 C/ 12 C and 14 C/ 12 C in the ocean mixed layer is on the order of a decade, 2 orders of magnitude slower than for oxygen. Less widely appreciated is the fact that the equilibration timescale is quite sensitive to the speciation of dissolved inorganic carbon (DIC) in the mixed layer, scaling linearly with the ratio DIC/CO 2 , which varies inversely with atmospheric p CO 2 . Although this effect is included in models that resolve the role of carbon speciation in air-sea exchange, its role is often unrecognized, and it is not commonly considered in the interpretation of carbon isotope observations. Here we use a global three-dimensional ocean model to estimate the redistribution of the carbon isotopic ratios between the atmosphere and ocean due solely to variations in atmospheric p CO 2 . Under Last Glacial Maximum (LGM) p CO 2 , atmospheric Δ 14 C is increased by ~30‰ due to the speciation change, all else being equal, raising the surface reservoir age by about 250 years throughout most of the ocean. For 13 C, enhanced surface disequilibrium under LGM p CO 2 causes the upper ocean, atmosphere, and North Atlantic Deep Water δ 13 C to become at least 0.2‰ higher relative to deep waters ventilated by the Southern Ocean. Conversely, under high p CO 2 , rapid equilibration greatly decreases isotopic disequilibrium. As a result, during geological periods of high p CO 2 , vertical δ 13 C gradients may have been greatly weakened as a direct chemical consequence of the high p CO 2 , masquerading as very well ventilated or biologically dead Strangelove Oceans. The ongoing anthropogenic rise of p CO 2 is accelerating the equilibration of the carbon isotopes in the ocean, lowering atmospheric Δ14C and weakening δ 13 C gradients within the ocean to a degree that is similar to the traditional fossil fuel “Suess” effect.