Detection of the Anthropogenic CO2 Signal in the Indian Ocean

Principal Investigator: Tsung-Hung Peng
Collaborating scientist:
Richard Wanninkhof, AOML
Objective: The oceanic uptake of anthropogenic CO2 is mainly estimated by mathematical models of the global ocean which are calibrated with the distribution of geochemical tracers. To verify such model estimates, we need to make field measurements to show the actual increase of carbon in the ocean as a result of CO2 uptake. The detection of anthropogenic CO2 signal is the first step in reaching this goal.
Rationale: The detection of anthropogenic CO2 signal in the ocean is hampered by the relatively small magnitude of the annual increase in total dissolved inorganic carbon (DIC) in seawater (~1 umol/kg , as compared with background DIC concentration of ~2000 umol/kg) and by the lack of precision in the measurements in the past. With the improvement in DIC analyses technique in last few years, it has now become possible to detect the anthropogenic CO2 signal on decadal time scales.
Method: The change in specific CO2 inventory can be estimated by comparing results of a recent NOAA-OACES survey cruise, I8N Repeat (I8NR) in 1995, with GEOSECS survey in 1978 in the Indian Ocean. After determining the systematic error between the two cruises, elimination of contribution from respiration of organic matter by using Apparent Oxygen Utilization (AOU) and Redfield ratios, and the correction for possible alkalinity changes due to carbonate dissolution, the salinity normalized total DIC concentrations along isopycnal surfaces, representing the upper thermocline waters, are compared. The significant increase in DIC from GEOSECS to I8NR represents the anthropogenic CO2 signal for the period between 1978 and 1995.
Accomplishment: An increase of 11 +/- 4.5 umol/kg at potential density of 26.6 is observed, with smaller increases on denser isopycnal horizons. The signal is undetectable near density surface of 27.2. The mean DIC inventory increase in the water column during this period is estimated to be about 0.5 umol/kg per year. The mean CO2 uptake in the latitude zone between 20o S and 5o N in the Indian Ocean is estimated to be 1.2 x 1014 mol each year (or 0.1 GtC/yr).
Key reference:
Peng, T.-H., R. Wanninkhof, J. Bullister, R. Feely, and T. Takahashi, Anthropogenic CO2 signal observed in the Indian Ocean, Tellus. (submitted).

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