Caldeira Lab Research:Energy, Global Carbon Cycle, and Climate/Ocean acidification and ocean carbon cycle

Carbon isotope constraints on vertical mixing and air-sea CO2 exchange

S. J. Fallon, T. P. Guilderson, & K. Caldeira

One of the strongest constraints on the exchange and uptake of anthropogenic carbon dioxide (CO2) by the ocean is derived from systematic and secular variations of the carbon isotopic signature of the dissolved inorganic carbon (DIC) pool as a consequence of the burning of 13C and 14C - depleted fossil fuel CO2.

Fallon, S.J., T.P. Guilderson, and K. Caldeira, Carbon isotope constraints on vertical mixing and air–sea CO2 exchange, Geophysical Research Letters 30, Art No. 2289, 2003.

Figure 2: Southwest Pacific coral and sclerosponge Δ14C records to time.


We have developed a ∼45 year carbon isotope record (δ13C, Δ14C) from the coralline sponge Acanthochaetetes wellsi from Vanuatu in an effort to examine air-sea CO2 exchange using both the δ13C Suess effect and the bomb-14C transient. From 1953 to 1999 δ13C decreased by 0.9‰. Pre-bomb Δ14C is -59‰, consistent with coral based estimates from the same region and the post-bomb maximum (+121‰) is achieved in 1973. A 1-D box-diffusion model was employed to quantify vertical diffusivity and air-sea exchange rates. The model suggests that a low vertical diffusion rate (0.1 cm2 s-1) coupled with a moderate CO2 exchange rate produces the overall observed shape of the pre-post bomb Δ14C record and the general large scale features of the δ13C time series. These parameters are on the low end of values used in ocean-carbon GCMs but are consistent with microscale tracer experiments.