Mineral ballast and particle settling rates in the coastal upwelling system off NW Africa and the South Atlantic

Abstract

The ocean off NW Africa is the second most important coastal upwelling system with a total annual primary production of 0.33 Gt of carbon per year (Carr in Deep Sea Res II 49:59-80, 2002). Deep ocean organic carbon fluxes measured by sediment traps are also fairly high despite low biogenic opal fluxes. Due to a low supply of dissolved silicate from subsurface waters, the ocean off NW Africa is characterized by predominantly carbonatesecreting primary producers, i.e. coccolithophorids. These algae which are key primary producers since millions of years are found in organic-and chlorophyll-rich zooplankton fecal pellets, which sink rapidly through the water column within a few days. Particle flux studies in the Mauretanian upwelling area (Cape Blanc) confirm the hypothesis of Armstrong et al. (Deep Sea Res II 49:219-236, 2002) who proposed that ballast availability, e.g. of carbonate particles, is essential to predict deep ocean organic carbon fluxes. The role of dust as ballast mineral for organic carbon, however, must be also taken into consideration in the coastal settings off NW Africa. There, high settling rates of larger particles approach 400 m day À1 , which may be due to a particular composition of mineral ballast. An assessment of particle settling rates from opal-production systems in the Southern Ocean of the Atlantic Sector, in contrast, provides lower values, consistent with the assumptions of Francois et al. (Global Biogeochem Cycles 16(4):1087. Satellite chlorophyll distributions, particle distributions and fluxes in the water column off NW Africa as well as modelling studies suggest a significant lateral flux component and export of particles from coastal shelf waters into the open ocean. These transport processes have implications for paleoreconstructions from sediment cores retrieved at continental margin settings.

Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

“…By containing strong redox gradients in relatively narrow proximity, and by providing nutrients and trace metals, particles might strongly influence biogeochemical cycles. It is well known that a pronounced POM/particle-enriched turbid layer (a so-called intermediate nepheloid layer) exists in the core of OMZs adjacent to coastal upwelling regions, such as those found off Peru, Mauritania and the Arabian Sea (see, e.g., Stramma et al, 2013;Naqvi et al, 1993;Fischer et al, 2009).…”

Eolian dust influx and massive whitings during the kozlowski/Lau Event: carbonate hypersaturation as a possible driver of the mid-Ludfordian Carbon Isotope Excursion

“…Because the suspended/precipitated detrital compounds play an important role in the ballasting of the OM (Armstrong et al 2001, Klaas & Archer 2002, eolian dust may additionally increase the export of the OM from the surface to the depths (Fischer et al 2009, Gabbott et al 2010, favouring the formation of anoxic/euxinic bottom waters. The reduction of the mainly solid, insoluble Fe(III) oxides into well-soluble and highly bioavailable Fe(II) compounds in an anoxic environment has a positive feedback on surface bioproductivity.…”

Global database of surface ocean particulate organic carbon export fluxes diagnosed from the <sup>234</sup>Th technique

“…Some of these areas are deemed to be high production and export regions due to the occurrence of upwelling. export for the Mauritanian upwelling suggest that POC flux can peak at 5 to 25 mg m −2 d −1 (Fischer et al, 2009), and is therefore presumably higher in the upper water column. Also, in the Benguela system POC export has been estimated to be 550 mg m −2 d −1 on the basis of nutrient uptake (Waldron et al, 1992).…”