Downward particle fluxes of biogenic matter and Saharan dust across the equatorial North Atlantic

Abstract

Abstract. Massive amounts of Saharan dust are blown from the coast of northern Africa across the Atlantic Ocean towards the Americas each year. This dust has, depending on its chemistry, direct and indirect effects on global climate which include reflection and absorption of solar radiation as well as transport and deposition of nutrients and metals fertilizing both ocean and land. To determine the temporal and spatial variability of Saharan dust transport and deposition and their marine environmental effects across the equatorial North Atlantic Ocean, we have set up a monitoring experiment using deep-ocean sediment traps as well as land-based dust collectors. The sediment traps were deployed at five ocean sites along a transatlantic transect between north-west Africa and the Caribbean along 12° N, in a downwind extension of the land-based dust collectors placed at 19° N on the Mauritanian coast in Iouîk. In this paper, we lay out the setup of the monitoring experiment and present the particle fluxes from sediment trap sampling over 24 continuous and synchronized intervals from October 2012 through to November 2013. We establish the temporal distribution of the particle fluxes deposited in the Atlantic and compare chemical compositions with the land-based dust collectors propagating to the downwind sediment trap sites, and with satellite observations of Saharan dust outbreaks. First-year results show that the total mass fluxes in the ocean are highest at the sampling sites in the east and west, closest to the African continent and the Caribbean, respectively. Element ratios reveal that the lithogenic particles deposited nearest to Africa are most similar in composition to the Saharan dust collected in Iouîk. Downwind increasing Al, Fe and K contents suggest a downwind change in the mineralogical composition of Saharan dust and indicate an increasing contribution of clay minerals towards the west. In the westernmost Atlantic Ocean, admixture of re-suspended clay-sized sediments advected towards the deep sediment trap cannot be excluded. Seasonality is most prominent near both continents but generally weak, with mass fluxes dominated by calcium carbonate and clear seasonal maxima of biogenic silica towards the west. The monitoring experiment is now extended, with autonomous dust sampling buoys for better quantification of Saharan dust transport and deposition from source to sink and their impact on fertilization and carbon export to the deep ocean.

Seasonal provenance changes of present-day Saharan dust collected on- and offshore Mauritania

“…16 Scheuvens et al (2013). transport (Delany et al, 1967;Glaccum and Prospero, 1980;Chester and Johnson, 1971b;Schütz and Sebert, 1987). A strong downwind decrease in quartz content in Saharan dust was also observed by Korte et al (2017).…”

“…This decrease in particle size between the stations CB and CBi was observed before and was attributed to the preferred gravitational settling of coarse particles during dust transport (Friese et al, 2016). Moreover, many studies have confirmed a downwind fining of the terrigenous fraction of surface sediments off NW Africa (Koopmann, 1981;Holz et al, 2004;Fütterer, 1980;Radczewski, 1939;Lange, 1975), and it is intuitively logical.…”

Seasonal provenance changes in present-day Saharan dust collected in and off Mauritania

“…Further, rutile was also present in the silt and sand fraction of Saharan dust sampled perpendicular to the coast on the research vessel (Chester and Johnson, 1971b). Palygorskite was found in the clay fraction of the surface sediment of sediment core GIK12329 (19 • 22 N, 19 • 56 W) off Cap Blanc and is considered a characteristic mineral of Saharan dust (Lange, 1975). The observed annual average C / K ratio (C / K = 4) recorded for the bulk size fraction of the trap samples was larger than the C / K ratio (C / K = 0.3-1) recorded in the clay fraction of surface sediment samples off Cap Blanc by Lange (1982).…”

“…2). The principle of particle collection is much the same as described by Van der Does et al (2016) and Korte et al (2017). At the top of the opening, a honeycomb grid is installed to prevent large swimmers (> 1 cm) from entering the trap.…”

“…Moreover, kaolinite is usually considered indicative of tropical weathering and the laterites of the southern Sahara and Sahel (Lange, 1975(Lange, , 1982Biscaye, 1964). Outcrops of quaternary laterites as well as outcrops of lower Eocene horizontal layers of palygorskite and sepiolite were described near Thiès in Senegal (García-Romero et al, 2007).…”