From Desert to Jungle: How Sahel Dust Sustains Amazonian Life

Without the African Sahara, the Amazon would be a desert as well. While intense precipitation and the ensuing floods wash most soluble minerals from the rainforest soil, mineral dust blown from the Sahara provides elemental nutrients such as aluminum, silicon, iron, titanium, and manganese (Al, Si, Fe, Ti, and Mn, respectively). Ben-Ami et al. (2010) combined satellite data with on-site analyses to better understand exactly how much dust from the Bodélé region of the Sahara reaches the Amazon, at what times, and in what routes. They examined two major dust events both in February 2008 and concluded that the Bodélé dust takes an average of 10 days to reach the Amazon forest canopy and that it mixes with both marine aerosols and biomass-burning aerosols along the way. The aerosol layer had more vertical range than expected as well, from 3 km above ground to the boundary layer, the more turbulent air layer below the cloud level. Such concrete data on the evasive study of global dust flux patterns allows scientists to better comprehend not only global climate patterns, but the dependency of biota on mass geochemical processes.— Elise Wanger
Ben-Ami, Y., Koren, I., Rudich, Y., Artaxo, P., Martin, S., Andreae, M., 2010. Transport of North African dust from the Bodélé depression to the Amazon Basin: a case study. Atmospheric Chemistry and Physics 10, 7533–7544.

The Bodélé has a unique combination of geological and meteorological qualities conducive to supplying the Amazon with nutrient elements. The estimated 133,532 km2 Bodélé depression sits in a valley which lake Mega-Chad formerly extended into in the Holocene several thousand years ago. Thus the soil contains SiO2, Al2O3, and Fe due to the diatomite and diatomite sand. Diatoms, single-celled algae that grow in massive numbers, leave elemental nutrients in the form of fossil deposits on lake bottoms, and the diatomaceous earth or sedimentary rock that contain these elements are known at diatomite. This uniquely signatured soil gets picked up by the Harmattan trade winds arriving from the northeast, which cross directly over the region. As these winds enter the Bodélé, the funnel effect of the valley narrows the passage of air and causes an acceleration of persistent high winds, perfect for dust transport. The Harmattan winds are seasonal, prevailing from December to mid-March, when the pressure along the equator is lowest, amplifying the gradient between regions, which is why the dust deposition in the Amazon occurs during the wet season. During the dry season, biomass smoke will often precede crustal dust particles as a prelude to the wet season, although as of yet no biological benefit has been found related to smoke aerosols on terrestrial life. For the 2003–2004 winter and spring seasons, researchers Koren et al. (2006) estimated that the Bodélé depression emitted (58±8) ´ 106 tons of dust, corresponding to over 7´105 tons per day. Although this number is significantly higher than the 1´106 tons observed by Ben-Ami et al. in regards to the February 2008 dust events (which only calculates a half-day of emission), both studies elucidate just how extensive the transport and deposition of Bodélé dust emissions can be.
The ground measurements of soil took place between February 7 and March 14, 2008, in a relatively unmanipulated forest of Brazil. The elemental analyses and proportion of Bodélé sediments were used to confirm the satellite findings. Polycarbonate filters above the canopy were installed in same location to analyze aerosol content as well, which gave a better sense of how much wind-blown particulate was biomass-burning aerosols or seawater particulates instead of dust. The surface wind speed and the direction of the wind were both calibrated using resolution imaging and spectroradiometry from a daytime and evening-time satellite. A spectroradiometer measures the radiation emitted per unit wavelength, in this situation being 440 nanometers (nm), which provides a resolution of 1 km2 per pixel. The Aerosol Optical Depth (AOD) was measured using the same “evening” satellite at 550 nm, providing a 10 km2/pixel resolution. The optical depth measures the transparency of an object by calculating how much radiation travels through it by looking at the intensity of the photons (the amount of light), the distance the light travels, and the amount of light backscattered or absorbed. The more light that travels through the material, the lower the AOD. In areas without clouds and few other particulate matters, the AOD can be assumed to be almost completely an analysis of dust, and the mass can therefore be easily calculated as a matter of the volume of the air column multiplied by a coefficient compensating for the influence of aerosols such as air pollutants on the AOD. Ben-Ami et al. chose a coefficient correlating to levels of other aerosols during moderate dust events, potentially making their estimates of the Bodélé region dust events overly conservative. Yet even veering on the safe side, the AOD of the dust route regions during a dust event is quite high and calibrates to an astounding mass of between 1´105–1´106 tons of emissions in the Bodélé from early morning to 12:30 P.M.
The fraction of aerosols with various diameters was also measured over satellites at 550 nm and through applying the Angström exponent to the hard data. The Angström exponent describes the effect of wavelength on the AOD, so that the AOD can be calibrated for any frequency of light. The lower the Angström exponent, the less wavelength level changes the amount of absorption and backscatter. Since large particles change less with wavelength, calculating the respective Angström exponent can indirectly measure the particulate size of the dust. Biomass-burning aerosols and marine aerosols from sea salts and organic matter were measured and subtracted from the total AOD in areas with more potential particulates, identified by their respective sizes (all under 1 mm).
The segregation of dust from clouds in the data was determined by calculating the “volume depolarization ratio” (VDR; Ben-Ami et al. 2010; 7535) with a satellite that uses infrared rays to capture cloud aerosol levels through measuring the polarization of the rays. As light hits a surface, it will usually backscatter, meaning that it will reflect back in the same direction it came from. This is typically the case with symmetrical particles. But less symmetrical particles, like dust, will reflect some of the waves perpendicular to the direction from which the wave derived. So while clouds, having fairly spherical particles will have a relatively low VDR, dust will have quite a high VDR. Looking at the different backscatter patterns themselves also help separate satellite data for dust versus clouds. Clouds have a strong backscatter signal with different vertical dimensions and horizontal dimensions.
Once the actual dust measurements could be calibrated—after correcting for other aerosols and cloud cover—Ben-Ami et al. could calculate the mass, rate, and elemental composition of the Bodélé dust from source to sink. Forward trajectories from the satellite data show that the crustal elements in the Amazon take between 10–17 days to arrive from the Bodélé, and back trajectory calculations show that the Bodélé dust takes about 2.5 days to reach the AERONET station in south-central Nigeria. According to the decreasing VDR over the course of the route, significant sedimentation of the dust occurs over the ocean along the way, more than doubling dust loading over the Atlantic. The VDR also indicated that many regions had a value right in between that expected for biomass smoke and that of dust, meaning the two aerosols are probably mixing in the trade winds. This mixing increased over the course of the dust event; for some reason more dust seems to dissipate towards biomass-burning regions. As expected, from the soil analysis on-site, Ben-Ami et al. observed more than tenfold more Si, Al, Fe, Mn, and Ti from the 18–19 of February dust deposition period. They also detected a chlorine content, apparently a result of dust mixing with the sea salt over the ocean before depositing.

The results from the study of Ben-Ami et al. lack any major conclusions, and an overall mass cannot be determined given the ±30% uncertainty in the calculation. As the article recommends, further satellite analyses and more extensive physical data should be collected, especially since dust deposition levels may shift with climate change.

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