Oxygen and carbon stable isotope tracers of the water masses in the Central Brazil Basin

Abstract

Stable isotope characteristics of the six water layers in the Central Brazil Basin are defined and compared to the various sources from which they originate. The ∂ 18 O−S‰ relationship allows us to discriminate between the more 18O-rich/more saline surface waters and South Atlantic Central Water formed in the tropical-subtropical zones, and the low- 18O/fresher water layers (Antarctic Intermediate Water: AAIW, Circumpolar Water: CPW, North Atlantic Deep Water: NADW, Antarctic Bottom Water: AABW) generated at high latitudes. The ∂ 18O-∂ 13C relationship emphasizes the specificity of each water mass: the upper water layers, where evaporation and photosynthesis are active, are the more 18O− 13C-rich waters (+0.3< ∂ 18 O<+1.5; +1.0< ∂ 13C<+2.0), whereas intermediate, deep and bottom waters, where fresh-water dilution and organic matter oxidation are significant, have the lowest 18O− 13C contents (−0.1< ∂ 18 O<+0.5; +0.5< ∂ 13C<+1.1). Aging of the NADW in this area of the southwest Atlantic results in an average ∂ 13C decrease of about 0.35‰. However, discrimination of water masses by their oxygen and carbon isotope compositions may become more difficult when they are far from their source area and when lateral mixing has occurred; this is the case for the CPW layer in the Central Brazil Basin.