Oceanic radiocarbon between Antarctica and South Africa along WOCE Section16 at 30 degrees E

Accelerator mass spectrometry (AMS) radiocarbon measurements were made on 1 20 samples collected between Antarctica and South Africa along 30 degrees E during the WOCE-France CIVA1 campaign in February 1993. Our principal obje ctive was to complement the Southern Ocean's sparse existing data set in or der to improve the C-14 benchmark used for validating ocean carbon-cycle mo dels, which disagree considerably in this region. Measured C-14 is consiste nt with the theta-S characteristics of CIVA1. Antarctic Intermediate Water (AAIW) forming north of the Polar Front (PF) is rich in C-14, whereas surfa ce waters south of the PF are depleted in C-14. A distinct old C-14 Signal was found for the contribution of the Pacific Deep Water (PDW) to the retur n flow of Circumpolar Deep Waters (CDW). Comparison to previous measurement s shows a C-14 decrease in surface waters, consistent with northward displa cement of surface waters, replacement by old deep waters upwelled at the An tarctic Divergence, and atmospheric decline in C-14. Conversely, an increas e was found in deeper layers, in the AAIW. Large uncertainties, associated with previous methods for separating natural and bomb C-14 when in the Sout hern Ocean south of 45 degrees S, motivated us to develop a new approach th at relies on a simple mixing model and on chlorofluorocarbon (CFC) measurem ents also taken during CIVA1. This approach leads to inventories for CIVA1 that are equal to or higher than those calculated with previous methods. Di fferences between old and new methods are especially high south of approxim ately 55 degrees S, where bomb C-14 inventories are relatively modest.