Dust source transfer from North Africa to the Amazon Basin: geochemical constraints on their long-term sources and composition

At a global scale, dust can serve as a vector for transferring elements from nutrient-rich soils to nutrient-depleted ecosystems, acting as a natural fertilizer [1]. The Amazonian rainforest, which is partly developed over nutrient-poor lateritic soils, illustrates this concept by receiving annually 8.5 Tg of dust from North African regions [2]. This phenomenon is well-documented and captured by both satellite-derived and in situ observations; however, the documentation of the long-term dust sources in North Africa and their associated chemical composition remains debated today [3,4]. This study presents two chronicles of dust collected at the Atmospheric Tall Tower Observatory (ATTO) during the dust-active season (February to April) in 2016 and 2017. Following a chemical extraction procedure already reported elsewhere [5], the chemical compositions and Sr-Nd-Pb isotope signatures of samples collected during low-dust conditions and dust outbreak events have been analyzed.

Following a statistical ACP and clustering analysis, the extracted water-soluble, acid-soluble, and residual fractions show that dust loading is the main driver of aerosol composition. Carbonated minerals do not survive efficiently in the atmospheric conditions encountered during transatlantic transport within the Saharan Air Layer and are readily solubilized. Most of the silicates and oxides are resistant to atmospheric chemical weathering, with the exception of poorly crystallized Al-Fe oxides. Finally, the geochemical signals of trace metals, potassium, and phosphorus can be complicated by anthropogenic particles or emitted bioaerosols, in addition to dust.

Predominant north African dust sources are identified by combining rare earth element patterns with Sr-Nd-Pb radiogenic isotopes, both of which are clearly diagnostic. A Bayesian mixing model (MixSIAR) is also used to quantify the long-term proportion of each source, while satellite products (CALIPSO, MERRA-2) and back trajectory analyses (HYSPLIT) are used to confirm our observations. Western African soils characterized by alluvial deposits in wadis developed over Phanerozoic terrains are the dominant dust sources (55-90%), while soils associated with Precambrian cratonic areas can act sporadically during significant dust events. As already postulated using a satellite-derived model [3], the Bodélé Depression’s impact on dust reaching the Amazon Basin is negligible, despite its status as the dustiest place on Earth. These results are consistent with conclusions drawn for the Northern Hemisphere, particularly for the Caribbean [5], although dust transport and atmospheric conditions over North Africa differ seasonally (between boreal winter and boreal summer). Finally, the chemical composition of the dust measured for all dust events reaching ATTO in 2016 and 2017 is remarkably uniform and consistent with 2024 and 2025 collected samples from French Guiana and ATTO (Collignon et al., in prep.), allowing for a preliminary estimate of a long-term “averaged North African dust” composition reaching the Amazon Basin.

[1] Reicholf (1986), SNFE, 21, 251-255.

[2] Kok et al. (2021), ACP, 21, 8169-8193.

[3] Yu et al. (2020), GRL, e2020GL088020.

[4] Barkley et al. (2022), GRL, e2021GL097344.

[5] Kumar et al. (2018), EPSL, 487, 94-105.