Unraveling the geochemical signals from major episodes of Saharan dust at two different locations in the Amazon basin.

Desert dust is the most abundant aerosol by mass in Earth’s atmosphere (global dust loading of 22-29 Tg; [1]). One key region of interest is the Amazon Basin, which acts as a major sink for mineral dust transported from North Africa (deposition flux of ∼10 Tg.yr^-1; [1]), impacting the nutrient supply to this rainforest ecosystem [2]. Currently, Western African sources are expected to be the predominant dust source based on previous geochemical studies [3] and atmospheric modeling [4], while the contribution of the Bodélé region is highly debated [4]. However, further constraints are still needed to elucidate the nutrient bioavailability associated with dust and other aerosol types, as well as how chemical transformations may affect the dust geochemical signal during transport and continentalization.

This study focuses on simultaneous high-resolution records of North African dust episodes reaching two different South American locations from January to March 2025. The first location is a coastal observatory in French Guiana (ATMO), while the second is located in the central Amazon forest, in Brazil (ATTO). Although these observatories are separated by more than 1,000 km, they are both influenced by similar transatlantic air mass trajectories, enabling an assessment of the impact of air mass continentalization on the chemical and physical characteristics of the aerosol particles. Aerosol samples have been chemically characterized using a recently developed selective extraction protocol [3], which segregates particles into water-soluble, acid-soluble, and residual material, including the silicate fraction of dust [5].

A 65 % dust loading reduction is observed between ATMO and ATTO sites, accompanied by a decrease in the soluble fraction from 20–50 %, dominated by sea salt at ATMO, to less than 10 % at ATTO. Other constituents originate from the dissolution of carbonates (Ca, Mg) due to atmospheric processes, from the leaching of soot particles or the emission of bioaerosols (K, P), and from the partial dissolution of poorly crystallized oxides (Al, Fe).  

The silicate fraction, which dominates the aerosol mass (50-98%), reveals a remarkable stability in the elemental composition of dust, irrespective of the observatory location, the position within the dust event (onset, peak, or decay), or the meteorological conditions. This compositional consistency exhibits a highly coherent signal when compared with previous dust episodes observed in 2016, 2017, and 2024 [3]. Furthermore, isotopic signatures of Sr, Nd, and Pb, known as efficient proxies for dust sources, are in strong agreement with those measured during these earlier episodes, confirming the dominant role of the West African dust source and the negligible contribution of the Bodélé Depression. Overall, these findings underscore the robust stability of the geochemical signal carried by dust, thereby enhancing our understanding of the average dust composition that reaches the Amazon Basin. In contrast, the focus on more labile components is strategic since these elements are preferentially redistributed into the water- and acid-soluble fractions.

 

[1] Kok et al. (2021), https://doi.org/10.5194/acp-21-8169-2021

[2] Swap et al. (1992), https://doi.org/10.1034/j.1600-0889.1992.t01-1-00005.x

[3] Collignon et al., submitted.

[4] Yu et al. (2020), https://doi.org/10.1029/2020GL088020

[5] Kumar et al. (2018), https://doi.org/10.1016/j.epsl.2018.01.025