Particle size traces modern Saharan dust transport and deposition across the equatorial North Atlantic

Abstract

<p><strong>Abstract.</strong> Mineral dust has a large impact on regional and global climate, depending on its particle size. Especially in the Atlantic Ocean downwind of the Sahara, the largest dust source on earth, the effects can be substantial but are poorly understood. This study focuses on seasonal and spatial variations in particle size of Saharan dust deposition across the Atlantic Ocean, using an array of submarine sediment traps moored along a transect at 12˚ N. We show that the particle size decreases downwind with increased distance from the Saharan source, due to higher gravitational settling velocities of coarse particles in the atmosphere. Modal grain sizes vary between 4 and 33 μm throughout the different seasons and at five locations along the transect. This is much coarser than previously suggested and incorporated into climate models. In addition, seasonal changes are prominent, with coarser dust in summer, and finer dust in winter and spring. Such seasonal changes are caused by transport at higher altitudes and at greater wind velocities during summer than in winter. Also the latitudinal migration of the dust cloud, associated with the Intertropical Convergence Zone, causes seasonal differences in deposition as the summer dust cloud is located more to the north, and more directly above the sampled transect. Furthermore, increased precipitation and more frequent dust storms in summer coincide with coarser dust deposition. Our findings contribute to understanding Saharan dust transport and deposition relevant for the interpretation of sedimentary records for climate reconstructions, as well as for global and regional models for improved prediction of future climate.</p>

The mysterious long-range transport of giant mineral dust particles

“…2B ). This is substantially less than in winter, generally increasing the probability of giant dust particles reaching the sampling sites, consistent with the higher number of larger dust particles found in submarine sediment traps at our sampling sites in summer ( 4 ). As in winter, we would generally assume giant particles to only reach the buoys during the strongest wind situations as reflected in our trajectory computations.…”

“…Here, we present new data from the same trans-Atlantic transect as van der Does et al . ( 4 ), this time collected directly from the atmosphere by Modified Wilson and Cooke (MWAC) samplers (see Materials and Methods), mounted on moored dust-collecting surface buoys at two stations in the tropical North Atlantic Ocean. The passive air samplers collected one discrete sample during periods between 2013 and 2016, comprising 281 to 432 days ( Table 1 ) at approximately 3 m above sea level.…”

“…In Europe, giant dust particles were found >4000 km from their Saharan source ( 2 ), and dust particles up to 300 μm in diameter were sampled during aircraft campaigns over northwestern Africa ( 3 ). In marine sediment traps, positioned underneath the main Saharan dust plume in the Atlantic Ocean, giant particles are dominated by large quartz particles >100 μm, found at distances up to 4400 km from the west African coast ( 4 ).…”

Climate models miss most of the coarse dust in the atmosphere

“…1 and 3 suggests that coarse dust also deposits out of the atmosphere less quickly than predicted in models. This finding is supported by measurements that found coarse dust particles are deposited at greater distances from dust-source regions than can be predicted by models or explained by current dust deposition theory (5,12). Our results further indicate that dust deposits too quickly in models, because model biases are larger relative to measurements taken farther from dust sources than those taken closer to dust sources (see table S2).…”

“…One such key impact of coarse dust is its effect on biogeochemistry and carbon sequestration (11). Coarse dust dominates the deposited dust mass flux (12) and, thus, affects the delivery of key micronutrients like iron to the ocean surface, which may in turn influence the uptake of carbon dioxide into the deep ocean (13). Another way coarse dust affects Earth system processes is through interactions with clouds and radiation (14).…”