Distribution of suspended particulate matter at the equatorial transect in the Atlantic Ocean

Studied oceanographic transect across the Equatorial Atlantic is considered as “screenshot” of suspended particulate matter distribution against a hydrographical background. The area of abnormal high suspended matter volume concentrations was found above the Sierra Leone Rise from top to bottom (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 process is located within the Northwest African upwelling area, since the 5 plankton exposed to the Saharan dust abundance form a significant number of aggregates lately transported equatorward via Canary Current. The intermediate nepheloid layer associated with the Deep Western Boundary Current was recorded from the American Slope at the 3200–3700 m to the depth of 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 was in accordance with polymodal 10 distribution — the 2–4 μm and the 8–13 μm modes. The registered rise in percentage 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).

Two cores of recently ventilated (with higher levels of F11) NADW from the northern hemisphere are advected along the boundary and east to the Mid-Atlantic Ridge (MAR) (Molinari et al., 1992). A DWBC shallow core is centered at about 1500 m and a deeper one -at about 3500 m. The upper core of high F11 content, limited by the 3.2 and 4.7 • C potential temperature 130 isotherms, is typically associated with the deeper core, which is limited by the 1.8 and 2.4 • C isotherms. The Ceara Rise blocks the flow of the DWBC waters with potential temperature below 1.8 • C to the equator, causing it to recirculate back to the north.
More dense Antarctic Bottom Water (AABW) flows northwars though Atlantic Ocean basins and generally lays underneath NADW at the bottom. The velocity field of AABW (water with σ 4 > 45.90 kg m −3 ) is influenced by the overlying DWBC (Rhein et al., 1998). The AABW consists of old deep-water masses (Reid, 1994) with negligible F11 concentrations. According 135 to Rhein et al. (1998), AABW bifurcates at the equator: roughly 30% of the northward AABW flow comes to the Guiana Basin to the Guiana Basin central part. The high level of F11 (> 0.07 pmol kg −1 ) from LNADW affects the closest to it AABW part -the one nearby the Ceara Rise, thus this AABW branch also experiences a rise in F11 content in comparison with the far eastern AABW branch. To the north from the Ceara Rise similar conditions are found (Rhein et al., 1998). According to (Whitehead Jr and Worthington, 1982;Rhein et al., 1998), the core of AABW is located to the east the Ceara Rise at 43.3 • W.
AABW mixes with LNADW in the fracture zones at equator, so AABW in the Eastern Atlantic differs from the similar water 145 in the Western Atlantic (Rhein et al., 1998). There are two important features for the Eastern Equatorial Atlantic (McCartney et al., 1991). First, the potential flow of AABW through the Kane Gap from the Sierra Leone Basin to the Gambia Abyssal Plain and back. Second, the potential AABW flow through the Vema Fracture Zone.
According to Arhan et al. (1998)  The distribution of SPM within the Ioffe-2000 transect is presented in Figure 3, while background hydrophysical and hydrochemical conditions were described in detail in (Sarafanov et al., 2007). The obtained volume SPM concentrations varied from 0.01 to 0.40 ppm within the Ioffe-2000 transect and up to 4.1 ppm at the stations at the Northwest African upwelling area.

Upper ocean
In the upper layers, most commonly, the vertical distribution of SPM has a surface maximum caused by primary production, 160 decreasing exponentially towards the deep. The main reason for this is the fact that the upper ocean contain both the external SPM sources (river discharge, atmospheric input) and the internal SPM sources (primary production) (Chester, 1990). Another reason for particle accumulation in the surface layer may be the pycnocline, which slows the sedimentation (Emelyanov, 2005).
The "sedimentation barrier" was presented by a visible pycnocline at depths of 50-150 m to the west of the 30 • W and 0-100 m to the east of this point on the Ioffe-2000 transect (Sarafanov et al., 2007). It was shown for the Eastern Equatorial Atlantic 165 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. The vast majority of particles in the upper open ocean waters is internal and is consists  (Smith et al., 1989).
The Ioffe-2000 transect did not reach the SPM-rich coastal waters, yet the coastal SPM source has caused local SPM maximum at the margins of the transect at a depth of 400-800 m (up to 0.18 ppm). It was the Amazon River that caused the far more pronounced influence of the American coast in comparison with the African one. It is well known that the surface 175 SPM transport from the Amazon River to the open ocean turns to the north-east alongside the coast (Gibbs, 1974). According to Sarafanov et al. (2007), the warm, high-saline and silicate-poor upper waters observed on the Ioffe-2000 transect above 400-500 m were advected mostly by the NBC. The NBC core was located immediately westward of the studied transect near the shelf break (Johns et al., 1998). A signature of the NBC eastern periphery is located between 46 • W and 47 • W at depths of 150-500 m (Sarafanov et al., 2007). The SMP concentrations within the NBC are relatively low. The main evidence of the 180 SPM supply from the Amazon River to the shelf and the continental slope is a huge sediment body -the Amazon Cone.
Bottom SPM maxima of Amazon River origin are able to form INL falling from the shelf break. INLs are mostly caused by the detachment of BNLs over the breaks in the slope (McCave et al., 2001). One of the INLs was apparently observed during the Ioffe-2000 transect as a significant rise in the SPM concentration below the NBC.