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

Abstract

Abstract. Saharan dust has a crucial influence on the earth climate system and its emission, transport and deposition are intimately related to, e.g., wind speed, precipitation, temperature and vegetation cover. The alteration in the physical and chemical properties of Saharan dust due to environmental changes is often used to reconstruct the climate of the past. However, to better interpret possible climate changes the dust source regions need to be known. By analysing the mineralogical composition of transported or deposited dust, potential dust source areas can be inferred. Summer dust transport off northwest Africa occurs in the Saharan air layer (SAL). In continental dust source areas, dust is also transported in the SAL; however, the predominant dust input occurs from nearby dust sources with the low-level trade winds. Hence, the source regions and related mineralogical tracers differ with season and sampling location. To test this, dust collected in traps onshore and in oceanic sediment traps off Mauritania during 2013 to 2015 was analysed. Meteorological data, particle-size distributions, back-trajectory and mineralogical analyses were compared to derive the dust provenance and dispersal. For the onshore dust samples, the source regions varied according to the seasonal changes in trade-wind direction. Gibbsite and dolomite indicated a Western Saharan and local source during summer, while chlorite, serpentine and rutile indicated a source in Mauritania and Mali during winter. In contrast, for the samples that were collected offshore, dust sources varied according to the seasonal change in the dust transporting air layer. In summer, dust was transported in the SAL from Mauritania, Mali and Libya as indicated by ferroglaucophane and zeolite. In winter, dust was transported with the trades from Western Sahara as indicated by, e.g., fluellite.

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

“…where F is the dust flux (g m −2 d −1 ), MAR is the mass accumulation rate (g d −1 ), A is the cross-sectional area of the inlet tube of the MWAC sampler (m 2 ) and n is the estimated sampling efficiency of the MWAC bottles. The sampling efficiency of the MWAC samplers is between 75 and 90 % for 30 µm dust (Goossens and Offer, 2000), which is within a similar size fraction of the Iouîk dust (Friese et al, 2017). For the vertical (downward) marine particle fluxes from the sediment traps, the samples were wet-sieved over a 1 mm mesh, wet-split into five aliquot subsamples using a rotary splitter (WSD-10, McLane Laboratories), washed to remove the HgCl 2 and salts, and centrifuged.…”

“…The spring high can be related to the tradewind intensities at lower altitudes, whereas the summer high points to sporadic dust storms, invisible on land by satellites. Backward trajectories reveal that the location in Iouîk is a transit area for long-range transported Saharan dust (Friese et al, 2017). Overall, the horizontal Saharan dust flux from the land-based MWAC samplers in Iouîk cannot be compared directly to the downward flux in residual mass to in the sediment traps.…”

“…This caused a downward movement of the traps as recorded by increased pressure (depth) by both CTDs, as well as deviations of up to 14 and 12 • from the vertical, respectively, as measured by the tilt meters, affecting three sample intervals at station M5 (intervals #8, 10 and 11). For these three intervals, we adjusted the total mass flux using F = F t − (1 + 1.4 sin 2θ ), with F being the vertical flux, F t the flux in the tilted trap, and θ the degree of tilt from the vertical (Gardner, 1985).…”

The ballasting effect of Saharan dust deposition on aggregate dynamics and carbon export: Aggregation, settling, and scavenging potential of marine snow

“…Dust collected in August was used for the high dust deposition experiment, and dust collected in April was used for the low dust deposition and scavenging experiment. We determined the grain‐size distribution of the two dust suspensions using a Beckmann Coulter laser particle sizer (LS 13 320) equipped with a Micro Liquid Module (MLM, analytical error of ± 1.26 μ m (± 4.00%), see Friese et al ). The peak in the particle diameter frequency (given in volume %) was at 30 μ m for both samples, which was similar to Saharan dust collected with deep sea sediment traps off Cape Blanc (Friese et al ).…”

Estimates of African Dust Deposition Along the Trans‐Atlantic Transit Using the Decadelong Record of Aerosol Measurements from CALIOP, MODIS, MISR, and IASI

“…Such in situ‐based climatologies of dust deposition are generally scarce and limited in temporal and spatial coverage. In this study, we compile a data set (see Table S1 in the supporting information for details) by taking from a widely used precompiled data set (Albani et al, ; and references therein) complemented by some recent observations in the studied region (Friese et al, ; Korte et al, ). See Table S1 in for details.…”