Detection of the Anthropogenic CO2
Signal in the Indian Ocean
Richard Wanninkhof, AOML
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.
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.
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.
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).
Peng, T.-H., R. Wanninkhof, J. Bullister, R. Feely, and T.
Takahashi, Anthropogenic CO2 signal observed in the Indian Ocean, Tellus.
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