Ventilation and anthropogenic CO2 in the Sulu Sea

The Sulu Sea, a semi-enclosed deep marginal basin, has limited exchange of deep water and poorly constrained ventilation time-scales. Here we use observations of the transient tracer CFC-12 to calculate the ventilation time-scale and the concentration of anthropogenic CO2 (Cant) in the Sulu Sea based on the transient time distribution (TTD) method. We found that CFC-12 has penetrated the entire water column in the Sulu Sea and the mean ages increase with depth monotonically and exceed 400 years below the depth of about 2500 m. The mean ages derived from CFC data in the Sulu Sea below about 500 m are lower than those in the South China Sea (SCS) at the same depth, indicating that the ventilation time of deep water is shorter in the Sulu Sea than the SCS. The mean column inventory of Cant in the Sulu Sea is 39 mol m− 2 (error range (ER): 30-49 mol m− 2) for the deeper part and the estimated total Cant inventory in 2011 is 81 Tg (ER: 66-98 Tg). By applying the Transient Steady State approximation on the Cant data we calculated the lower limit of the diapycnal diffusivity, Kz, to 1.1 × 10− 4 m2 s− 1 (ER: 0.7-1.8 × 10− 4 m2 s− 1) for the Sulu Sea below the surface mixed layer.