Abstract
Abstract. A suspended particulate matter distribution against a hydrographical background was studied at the oceanographic transect across the equatorial Atlantic in the year 2000. An area of abnormally high suspended matter volume concentrations was found above the Sierra Leone Rise in the entire water column (eastern part of the transect). The suggested explanation for the anomaly is based on the ballast hypothesis whereby solid particles are incorporated as ballast into suspended biogenic aggregates, leading to increased velocities of sinking. This occurs within the Northwest African upwelling area, where the plankton exposed to the Saharan dust abundance form a significant number of aggregates, which are later transported equatorward via the Canary Current. An intermediate nepheloid layer associated with the Deep Western Boundary Current was recorded from the South American Slope at depths of 3200–3700 to 4300 m above the Para Abyssal Plain. Antarctic Bottom Water enriched in suspended matter was found mostly in the troughs at 40–41∘ W. It was detached from the bottom, coinciding with the core of the flow due to the bottom rise “dam” located up-stream. The grain size of particles along the entire transect has a polymodal distribution with 2–4 and 8–13 µm modes. The registered rise in percentage in some parts of the transect of the 7–21 µm sized particles suggests the presence of the well-known coarse mode (20–60 µm) formed by aggregation of transparent exopolymer particles (mucus).
Highlights
Numerous studies have focused on characterizing the suspended particulate matter (SPM) during the past half a century
The distribution of SPM within the Ioffe-2000 transect is presented in Fig. 4, while background hydrophysical and hydrochemical conditions were described in detail in Sarafanov et al (2007)
It was shown for the Eastern Equatorial Atlantic that the largest particulate elemental and mass concentration gradient occurs at the base of the mixed layer, where the particle and organism maximums are located
Summary
Numerous studies have focused on characterizing the suspended particulate matter (SPM) during the past half a century. The oceanic SPM spread may be depicted as a three-layer structure with the following layers: (a) a surface water, (b) a clear water minimum, and (c) a deep water (Biscaye and Eittreim, 1977; McCave, 1986; Chester, 1990). It was indicated that the distribution of the SPM in oceans basically shows the interaction between the bottom sediments and abyssal waters movement, so SPM-rich benthic nepheloid layers (BNLs) coincide with pathways of the western boundary currents (Biscaye and Eittreim, 1977). The works of Armi (1978) and McCave (1983) pointed out the lateral advection of SPM, which occurs due to bottom mixed layers being detached from the slope and leads to thickening and layering of BNLs. There are strong intermediate nepheloid layers (INLs) in the clear water minimum layer in some regions (Thorpe and White, 1988)
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