Eric D. Galbraith (Montreal, Canada)
With 3 Figures
The equilibration timescale for a nonreactive gas (such as oxygen) in the mixed layer is on the order of weeks. In contrast, CO2 reacts with water, so that the dominant components of Dissolved Inorganic Carbon (DIC) are bicarbonate and carbonate. Because air-sea exchange of DIC must pass through the bottleneck of the tiny dissolved CO2 pool, its equilibration is more than an order of magnitude slower than nonreactive gases, typically requiring about a year. As a result, at much of the sea surface, the concentration of DIC – and its isotopes – are significantly out of equilibrium with the atmosphere. Modification of this disequilibrium could have played a significant role in glacial-interglacial changes – both in well-recognized, and obscure, ways.
The concentration of DIC in any parcel of water at the sea surface can be conceptualized as the sum of two components (Fig. 1). The first, and by far dominant component, is the sat-uration concentration, DICsat. This concentration is determined by the temperature, salinity, and alkalinity of the water, as well as the partial pressure of CO2 in the overlying atmosphere (pCO2). The difference between the saturation concentration and the actual concentration is defined as the ‘disequilibrium’ DIC, DICdiseq. Models suggest that the global preindustrial DICdiseq was fairly small, less than 3 % of DICsat (Ito and Follows 2013). However, this is partly due to the cancellation between the large positive DICdiseq of the Southern Ocean, where CO2-rich deep waters are brought only briefly to the surface before sinking, and nega-tive DICdiseq in the North Atlantic, where waters are undersaturated due to cooling and biolog-ical uptake prior to sinking. The global DICdiseq was almost certainly larger during the LGM.
The most well-known mechanism by which DICdiseq could have contributed to glacial carbon storage was suggested by Keeling and Stephens (2001). Because sea-ice gets in the way of air-sea exchange, an expansion of Antarctic sea ice across the Southern Ocean surface would have significantly increased the already large DICdiseq concentration of Southern Ocean waters. This must have occurred, although it is difficult to quantify due to uncertainties in the reconstructions of glacial sea ice extent and difficulty in simulating the dynamics of sea ice, the mixed layer, and deep water formation in the Southern Ocean.
Two additional factors could have further increased the global inventory of DICdiseq during the LGM. First, because most of the DICdiseq in the Southern Ocean surface is inherited from upwelled waters, themselves enriched in carbon by the soft-tissue pump during their sojourn in the abyss, any mechanism that increases the store of soft-tissue-pump carbon in the deep Southern Ocean (such as iron fertilization or deep remineralization) would have enhanced
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Fig. 1 DIC components at the surface (preformed) and in the subsurface (remineralized). Modified after Galbraith and Jaccard 2015.
DICdiseq. Second, a change in global water mass distribution, such as the ‘standing volume effect’ of Skinner (2009), could have increased the global amount of DICdiseq by expanding the volume of deep waters occupied by Southern-sourced waters rich in DICdiseq.
The pCO2 effect also has implications for the distributions of carbon isotopes in the ocean during the glacial and deglaciation. Because they are ratios, the equilibration timescale for δ13C and Δ14C is approximately one order of magnitude longer than for DIC itself, and varies linearly with DIC/CO2. This implies that the equilibration timescale for δ13C and Δ14C was a remarkable ~50 % longer during the LGM than it was during the preindustrial. This enhanced disequilibrium would have impacted each isotope system in its own way, as evaluated using a global 3-dimensional ocean model by Galbraith et al. (in review).
For radiocarbon, the slow equilibration caused by low glacial pCO2 would have caused
14C to build up in the atmosphere, all else being equal. The model simulations suggest this effect alone would have raised atmospheric Δ14C by ~30 ‰ (Fig. 2). As a result, the reservoir ages of the global ocean surface would have been, fairly uniformly, ~250 years higher than preindustrial during the LGM. The pCO2 effect would have varied linearly with 1/CO2 over the deglaciation. Meanwhile, expanded sea ice could have greatly slowed radiocarbon uptake by Southern Ocean surface waters, thereby decreasing the radiocarbon content of deep South-ern Ocean waters even with no change in circulation rates.
For the stable carbon isotopes, the model suggests that enhanced disequilibrium due to the pCO2 effect would have caused glacial Southern Ocean waters – charged with a heavy burden of poorly equilibrated, low-δ13C respired carbon – to have become lower in δ13C. As a result, the internal oceanic gradients of δ13C would have been amplified, increasing the differ-ence between North Atlantic and Southern Ocean waters on the order of 0.2 ‰ (Fig. 3). This effect would have contributed to the enhanced δ13C contrast between Northern and Southern source waters in the Atlantic (Curry and Oppo 2005). Again, expanded sea ice in the
South-The Role of Air-Sea Disequilibrium in Ocean Carbon Storage and its Isotopic Composition
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Fig. 2 Simulated glacial-interglacial changes in oceanic Δ14C, relative to the atmosphere, due only to the pCO2 effect on air-sea exchange. Modified after Galbraith et al. (in review).
Fig. 3 Simulated glacial-interglacial changes in oceanic δ13C, relative to the atmosphere, due only to the pCO2 effect on air-sea exchange. Modified after Galbraith et al. (in review).
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ern Ocean would likely have exacerbated the disequilibrium, further decreasing the δ13C of Southern Ocean waters.
It is important to point out that the pCO2 effect on the disequilibrium of the carbon iso-topes is resolved in models that include carbon speciation and changes in pCO2. However, the fact that the effect is often unrecognized, and interacts with other mechanisms of glacal-in-terglacial change, can obscure its role. In addition, it is difficult to reconstruct DICdiseq from observational records, hampering its identification in glacial CO2 dynamics.
During the deglaciation, the retreat of Southern Ocean sea ice and ~50 ppm rise of pCO2 during the interval 17.5 to 14.5 ka would have greatly reduced the air-sea disequilibrium of DIC and its isotopes. Among other effects, this would have shifted low δ13C out of the South-ern Ocean and into the upper ocean and atmosphere, amplifying the effect of a simultaneous weakening of the soft tissue pump.
References
Curry, W. B., and Oppo, D.: Glacial water mass geometry and the distribution of δ13C of ∑CO2 in the western Atlan-tic Ocean. Paleoceanography 20, doi:10.1029/2004PA001021 (2005)
Galbraith, E. D., and Jaccard, S. L.: Deglacial weakening of the oceanic soft tissue pump: global constraints from sedimentary nitrogen isotopes and oxygenation proxies. Quat. Sci. Rev. 109, 38 – 48 (2015)
Galbraith, E., Kwon, E. Y., Bianchi, D., Hain, N. P., and Sarmiento, J.: The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere and ocean. Global Biochem. Cycles (in review)
Ito, T., and Follows, M. J.: Air-sea disequilibrium of carbon dioxide enhances the biological carbon sequestration in the Southern Ocean. Global Biogeochem. Cycles 27/4, 1129 –1138 (2013)
Keeling, R. F., and Stephens, B. B.: Antarctic sea ice and the control of Pleistocene climate instability. Paleocea-nography 16/1, 112–131 (2001)
Skinner, L.: Glacial-interglacial atmospheric CO2 change: a possible standing volume effect on deep-ocean carbon sequestration. Clim. Past 5/3, 537–550 (2009)
Prof. Eric Galbraith, Ph.D.
McGill University
Department of Earth and Planetary Science 3450 University St.
Montreal, Quebec Canada H3A 2A7 Phone: +1 514 3983677 E-Mail: eric.galbraith@mcgill.ca
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