COMPARISON OF TWO APPROACHES TO QUANTIFY ANTHROPOGENIC CO2 IN THE OCEAN: RESULTS FROM THE NORTHERN INDIAN OCEAN.

Reference
Coatanoan, C, C. Goyet, N. Gruber, C.L. Sabine, and M. Warner, Comparison of two approaches to quantify anthropogenic CO2 in the ocean: Results from the northern Indian Ocean. Global Biogeochemical Cycles, 15(1), 11-25 2001.

Abstract
This study compares two recent estimates of anthropogenic CO2 in the northern Indian Ocean along WOCE 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 DC* approach first described by Gruber et al. (1996), whereas Goyet et al. (1999) used an optimum multi-parameter 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. The two methods estimate similar overall distributions of anthropogenic CO2, but substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO2 concentrations estimated using the DC* approach average 12 +/- 4 umol kg-1 higher in this depth range than concentrations estimated using the MIX approach. Below approximately 800 m, the MIX approach estimates slightly higher anthropogenic CO2 concentrations and a deeper vertical penetration. Despite this compensatory effect, water column inventories estimated by the DC* approach are generally about 30% higher than those estimated by the MIX approach. 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.

see related articles by Gruber [1998], and Gruber et al. [1996]