Gas exchange and CO2 flux in the tropical Atlantic Ocean determined from 222Rn and pCO2 measurements


  • William M. Smethie Jr.,

  • Taro Takahashi,

  • David W. Chipman,

  • James R. Ledwell


Measurements of 222Rn vertical profiles and pCO2 in the surface water and the atmosphere were made simultaneously in the tropical Atlantic ocean as part of the TTO/TAS program. The gas exchange rate or piston velocity was determined from the 222Rn profiles, and the ΔpCO2 between the surface ocean and the atmosphere was determiend from the pCO2 measurements. The net flux of CO2 across the sea-air interface was calculated from these two data sets. The piston velocity ranged from 1.4 to 6.9 m/d and was correlated with wind speed. The slope of piston velocity versus wind speed was estimated to be between 0.3 and 1.1 (m/d)/(m/s). The ΔpCO2 ranged from −35 μatm at 15°N, 55°W to +64 /zatm at 5°S, 28°W, with the zero ΔpCO2 isopleth located at about 10°N. The high ΔpCO2 values can be explained by lateral advection of surface water from the east with heating and biological consumption of CO2 and alkalinity during transit. The net flux of CO2 was into the ocean north of 10°N latitude with values reaching a maximum of 1.4 mol m−2 yr−1 at 15°N, 50°W. South of 10°N, the net flux was out of the ocean, reaching a maximum value of 2.7 mol m−2 yr−1 at 8°S, 28°W. The average net flux from 10°N to 10°S was 1.3 mol m−2 yr−1 out of the ocean, which is equivalent to 0.15 gigatons of carbon per year if the flux determined applied throughout the year.