Carbon pathways and export associated with the southern Benguela upwelling system: a re-appraisal

Abstract

A network of six NO₃-N-driven carbon pathways between the ocean, shelf and sediments was described in a paper published in 1992. A budgetary approach at the annual scale meant that, if three of the pathways were quantified, solutions could be found algebraically for those remaining. The network of pathways in the present study remains unchanged in principle, but in respect of long-term carbon sequestration, there has been a shift in emphasis from ocean to continental shelf. This results from an adjusted estimate for carbon exported seawards of the continental shelf, mainly owing to a re-examination of the typical offshore penetration of upwelling-derived water. Whereas the 1992 paper used a study based on grey-scale contrast, from Meteosat imagery, to designate a region up to and occasionally beyond the 2 000 m isobath where water of upwelling origin was present on a quasi-permanent basis, the present study used actual sea surface temperatures (SSTs) from NOAA imagery and found comparatively little water <17°C beyond the 350 m isobath. This adjustment has important implications for the quantification of offshore potential new production calculated from the derived relationship between SST and NO₃-N integrated over the nominal euphotic zone. The proportion of southern Benguela new production thought to be sequestrated at time-scales which constitute a long-term loss to the system remains at 70%, but it is now proposed that approximately 66% is incorporated in continental shelf sediments and 4% is lost below the offshore permanent thermocline. The remaining 30% is re-cycled over shorter time-scales within the southern Benguela system.