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

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 Δ¹⁴ C 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.