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

Abstract

Abstract. 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 32 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.

Tropical Rains Controlling Deposition of Saharan Dust Across the North Atlantic Ocean

“…Seasonality of dust deposition observed in the sediment traps is well defined across the Atlantic, with highest fluxes in summer and autumn, and lowest fluxes in winter and spring (Table ). The seasonal amplitude decreases downwind, from east to west, with largest seasonal differences observed at station M1, in line with the downwind decrease in seasonality observed in the particle size of the deposited dust (Van der Does et al, ). Satellites have also observed highest dust fluxes in the Atlantic in summer, although contrary to the observations in the sediment traps, the satellites observed lowest fluxes in autumn and reaches less far west (Yu et al, ).…”

“…This is about 1.5 times higher than the lithogenic fluxes observed close to the Canary Islands by Neuer et al () and Brust et al (), who observe average deposition fluxes of 10 and 9.5 mg·m −2 ·day −1 , and a maximum deposition flux in winter of up to 54 and 43 mg·m −2 ·day −1 , respectively. The episodic high fluxes in winter are likely due to dust transport at lower altitudes during this time and therefore deposited closer to the source (Van der Does et al, ), while the overall lower fluxes probably relate to the more northern position of the Canary Island traps, away from the center of the main dust cloud. In our sediment traps, dust deposition also decreases sharply toward the north, to 2.5 and 2.9 mg·m −2 ·day −1 at M2 in 2013 and 2014, and toward the west to 3.7 mg·m −2 ·day −1 at M3 in 2014, and 3.3 and 2.3 mg·m −2 ·day −1 at M4 in 2013 and 2014 (Table ).…”

“…Reported seasonality of dust concentrations and deposition is different on either side of the Atlantic, peaking in winter in the east (Bory et al, 2002;Fischer et al, 2016;Fomba et al, 2014;Neuer et al, 2004;Ratmeyer et al, 1999;Skonieczny et al, 2013), and in summer in the west (Jickells et al, 1998;Prospero et al, 2014). These differences can be related to seasonal atmospheric dynamics like the altitude at which the dust is transported, with dust at lower altitudes deposited close to the source in the east, while dust transported at higher altitudes can reach the remote western Atlantic ( Van der Does et al, 2016;Yu et al, 2019). In addition, most of the studies on the eastern side of the Atlantic were carried out at latitudes above the main dust cloud and the northern position of the ITCZ, which could result in the opposing seasonal signal compared to the western Atlantic.…”

Coccolithophore fluxes in the open tropical North Atlantic: influence of thermocline depth, Amazon water, and Saharan dust

“…This is consistent with previous observations of E. huxleyi bursting into a bloom within only a few days in response to nutrient availability and clear sky conditions in surface waters (Guerreiro et al, 2013). The high tolerance of this species to high levels of light has been considered crucial for its capacity to dominate coccolithophore assemblages (Nanninga and Tyrrell, 1996;Tyrell and Merico, 2004). Although to a lesser extent compared to E. huxleyi, other taxa that also increased during this April period include Helicosphaera spp., Rhabdosphaera spp., and Umbilicosphaera spp.…”

“…Van der Jagt et al (2017) report more abundant and faster-sinking aggregates when formed from a natural plankton community that has been exposed to Saharan dust deposition compared to less abundant and slowersinking aggregates when formed without dust. The same authors argue that such dust-influenced aggregates would become heavily ballasted with lithogenic material at the surface and hence without scavenging any additional particles during their settling.…”