Anthropogenic carbon dioxide in the South Atlantic western basin

Abstract

The meridional WOCE line A17 was conducted during the austral summer of 1994 parallel to the eastern South American coast, from 55°S to 10°S, where one of the main limbs of the North Atlantic Deep Water (NADW), i.e., the southward-flowing Deep Western Boundary Current (DWBC) is found. Full-depth profiles of pH, total alkalinity and total inorganic carbon were measured and checked with analytical CO 2 certified reference materials (CRMs), providing a high-quality dataset with good internal consistency for the CO 2 system parameters that is well suited for anthropogenic CO 2 (C ANT) estimation. For the first time in the western Atlantic basin the C ANT has been calculated using four independent approaches and results are compared. The methods considered are the CFC-based TTD method and the φC T°, TrOCA and ∆C* carbon system-based back-calculation methods. All four methods have produced C ANT distribution patterns that are in general good agreement: maximum concentrations of C ANT (50–60 μmol kg − 1 ) are predicted for the upper warm South Atlantic central waters from the tropical gyres, while the minima (~ 5 μmol kg − 1 ) are located in the old northward-flowing branch of Circumpolar Deep Water. There are, however, some discrepancies detected. The TrOCA method yields the highest overall [C ANT] values, even over the theoretical limit of C ANT saturation for 1994 in the upper layers. The ∆C* approach consistently yielded negative estimates of C ANT below 2800 dbar, even after correcting a reported − 8 μmol kg − 1 bias in the alkalinity measurements of the WOCE A17 line. The main overall difference between the four methods corresponds to the relative C ANT maximum associated with the lower limb of NADW: this structure is well identified in the φC T° and TTD methods but seems to disappear in the case of TrOCA and ∆C*. In agreement with other intercomparison studies of C ANT, the specific inventories are significantly higher (~ 45%) than those reported in the GLODAP database obtained from the ΔC* method. This suggests that the South Atlantic stores more C ANT than initially expected, particularly towards the southernmost tip of the WOCE A17 line, close to the Southern Ocean. The φC T°, TrOCA and TTD methods confirm an increasing tendency of C ANT specific inventories south from the Equator, while the ΔC* method shows a decreasing trend south from 35°S.