FY 2001

Comparison of two approaches to quantify anthropogenic CO₂ in the ocean: Results from the northern Indian Ocean

Coatanoan, C., C. Goyet, N. Gruber, C.L. Sabine, and M. Warner

Global Biogeochem. Cy., 15(1), doi: 10.1029/1999GB001200, 11–25 (2001)

This study compares two recent estimates of anthropogenic CO₂ in the northern Indian Ocean along the World Ocean Circulation Experiment cruise I1 [Goyet et al., 1999; Sabine et al., 1999]. These two studies employed two different approaches to separate the anthropogenic CO₂ signal from the large natural background variability. Sabine et al. [1999] used the ΔC* approach first described by Gruber et al. [1996], whereas Goyet et al. [1999] used an optimum multiparameter mixing analysis referred to as the MIX approach. Both approaches make use of similar assumptions in order to remove variations due to remineralization of organic matter and the dissolution of calcium carbonates (biological pumps). However, the two approaches use very different hypotheses in order to account for variations due to physical processes including mixing and the CO₂ solubility pump. Consequently, substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO₂ concentrations estimated using the ΔC* approach average 12 ± 4 µmol kg⁻¹ higher in this depth range than concentrations estimated using the MIX approach. Below ~800 m, the MIX approach estimates slightly higher anthropogenic CO₂ concentrations and a deeper vertical penetration. Despite this compensatory effect, water column inventories estimated in the 0-3000 m depth range by the ΔC* approach are generally ~20% higher than those estimated by the MIX approach, with this difference being statistically significant beyond the 0.001 level. We examine possible causes for these differences and identify a number of critical additional measurements that will make it possible to discriminate better between the two approaches.