Carbon export and sequestration in the southern Benguela upwelling system: lower and upper estimates

Abstract

Three independent studies of carbon export and sequestration in the southern Benguela upwelling system are presented. They were undertaken by Waldron (upwelling index), Monteiro (discrete upwelling centres – gate hypothesis model) and Swart (cross-shelf advection in bottom nelpheloid layers). The annual estimates were, 3.9×10¹³, 0.72×10¹³ and 8.6×10¹¹ gC, respectively. The lowest estimate was derived from a consideration of low frequency lateral carbon export in the bottom nepheloid layer and was thought likely to be an under-estimate. Taking into account high frequency episodic events, intermediate nepheloid layers and along isopycnal export of DOC at surface and intermediate depths was thought likely to result in a substantial upward revision. The remaining two estimates were considered to be an upper and lower estimate of carbon export and sequestration due to factors inherent in the methodologies. The upper estimate presents a two-dimensional system, integrated alongshore; the lower estimate sums a series of upwelling centres in order to obtain a system flux. The former is therefore a uniform extrapolation along the coast while the latter omits upwelling between the upwelling centres.