Basin-scale changes of total organic carbon profiles in the eastern South Atlantic

X A Alvarez-Salgado,F F Pérez,M D Doval,A F Ríos

doi:10.3989/scimar.2001.65n11

Abstract

Total organic carbon (TOC) samples were collected at 6 stations spaced ~800 km apart in the eastern South Atlantic, from the Equator to 45°S along 9°W. Analyses were performed by high temperature catalytic oxidation (HTCO) in the base laboratory. Despite the complex advection and mixing patterns of North Atlantic and Antarctic waters with extremely different degrees of ventilation, TOC levels below 500 m are quasi-constant at 55±3 µmol C l-1, pointing to the refractory nature of deep-water TOC. On the other hand, a TOC excess from 25 to 38 g C m-2 is observed in the upper 100 m of the permanently stratified nutrient-depleted Equatorial, Subequatorial and Subtropical upper ocean, where vertical turbulent diffusion is largely prevented. Conversely, TOC levels in the nutrient-rich upper layer of the Subantarctic Front only exceeds 9 g C m-2 the deep-water baseline. As much as 70% of the TOC variability in the upper 500 m is due to simple mixing of reactive TOC formed in the surface layer and refractory TOC in deep ocean waters, with a minor contribution (13%) to oxygen consumption in the prominent subsurface AOU maximum at 200-400 m depth.

Highlights

The key contribution of dissolved organic matter (DOM) to carbon cycling and the implications for the role of the oceans in the regulation of the anthropogenic CO2 excess have been issues of renewed interest during the last decade (Toggweiler, 1989; Bascatow and Maier-Reimer, 1991; Sambrotto et al., 1993; Legendre and Le Fèvre, 1995)
An excess of degradable DOM in surface ocean waters - generated in situ by biological processes (Kirchman et al, 1993; Thingstad et al, 1997) - is a commonly observed world-wide trend
Seems to be composed of a minor fraction of fastcycling, highly-labile material (Carlson and Ducklow, 1995), which constitutes the substrate for the microbial loop organisms (Azam et al, 1983; Kirchman et al, 1991)

Summary

Introduction

The key contribution of dissolved organic matter (DOM) to carbon cycling and the implications for the role of the oceans in the regulation of the anthropogenic CO2 excess have been issues of renewed interest during the last decade (Toggweiler, 1989; Bascatow and Maier-Reimer, 1991; Sambrotto et al., 1993; Legendre and Le Fèvre, 1995). An excess of degradable DOM in surface ocean waters - generated in situ by biological processes (Kirchman et al., 1993; Thingstad et al, 1997) - is a commonly observed world-wide trend 1993; Carlson et al, 1994; Pakulski and Benner, 1994; Chen et al, 1996) This DOM excess seems to be composed of a minor fraction of fastcycling, highly-labile material (Carlson and Ducklow, 1995), which constitutes the substrate for the microbial loop organisms (Azam et al, 1983; Kirchman et al, 1991). In areas where convective mixing follows summer stratification, accumulated DOM is locally exported downwards during winter mixing and contributes to the in situ oxygen consumption in subsurface waters (Copin-Montégut and Avril, 1993; Carlson et al, 1994). Eppley and Peterson’s (1979) concept of ‘new production’

Methods

Results

Conclusion