Argon concentration in the deep sea as an indicator of atmosphere/ocean CO2 coupling and thermocline mixing
Abstract
Box model atmospheric pCO2 values are systematically more sensitive to changes in the biological pump than are pCO2 levels predicted by GCMs. Toggweiler et al. [2003] showed that the difference lies in the degree of equilibration between the high latitude surface ocean and the atmosphere, i.e. how much "traction" the surface ocean has on the atmosphere. pCO2 measurements in the real ocean are unable to tell us which of these models best represents the real ocean, because high nutrient levels counterbalance low temperatures in high latitude surface waters. This is the reason why box models and GCMs both get the right atmospheric pCO2 over the present-day ocean. Dissolved argon concentrations in the deep ocean may be usable to diagnose the high latitude disequilibrium of the real ocean. Hamme and Emerson [2002] measured 0-3% undersaturation of argon, and 2% supersaturation of neon, in the Atlantic and Pacific deep ocean. High heat fluxes before subduction generate undersaturation, because gas exchange is unable to keep up with changing SST, while bubble injection tends to generate supersaturation. Atmospheric pressure is typically ~1% lower over the Southern Ocean where deep water is formed. Neon is more sensitive to bubbles and less sensitive to heat fluxes than argon, while pressure offsets affect both gases equally. Using the pair of gases we can estimate that disequilibrium alone would generate a deep ocean 1-2.5% undersaturated in argon. Traditional box model deep argons are within 0.1% of equilibrium, while GCMs generate 1-2% undersaturation. More data would be useful, but the data we have seem to be telling us that the real ocean is more like a GCM than a box model, supporting a lowered atmospheric pCO2 sensitivity to the biological pump and other high-latitude forcings. Argon saturation is exponential in temperature, while mixing generates linear covariation of properties. Saturated waters of different temperatures, mixed together, therefore generate a supersaturated argon solution. This mechanism generates a thermocline maximum of about 3% supersaturation in GCMs and box models where waters of dissimilar temperatures mix. In the GCM, this feature is most pronounced in the equatorial Pacific, where there is unfortunately no recent data to compare to. The closest Hamme and Emerson profile is near Hawaii, where the GCM doesn't show supersaturation either. Argon supersaturation may serve as an indicator of diapycnal mixing, integrated over the lifetime of the water mass, balanced against direct isopycnal ventilation. New measurements of the regional distributions of inert gas concentrations, and measurements of heavier inert gases, would be useful. Hamme, R.C., and S.R. Emerson, Mechanisms controlling the global oceanic distribution of the inert gases argon, nitrogen, and neon, Geophys. Res. Lett., 29 (23), doi:10.1029/2002GL015273, 2002. Toggweiler, J.R., A. Gnanadesikan, S. Carson, R. Murnane, and J.L. Sarmiento, Representation of the carobn cycle in box models and GCMs: 1. Solubility pump, Global Biogeochemical Cycles, 17 (1), doi:10.1029/2001GB001401, 2003.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFMOS22D..02A
- Keywords:
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- 1635 Oceans (4203);
- 4540 Ice mechanics and air/sea/ice exchange processes;
- 4568 Turbulence;
- diffusion;
- and mixing processes;
- 4806 Carbon cycling;
- 4820 Gases