Environmental factors controlling the seasonal variability in particle sizedistribution of modern Saharan dust deposited off Cape Blanc

摘要

The particle sizes of Saharan dust in marine sediment core records have been used frequently as a proxyfor trade-wind speed. However, there are still large uncertainties with respect to the seasonality of theparticle sizes of deposited Saharan dust off northwestern Africa and the factors influencing this seasonality.We investigated a three-year time-series of grain-size data from two sediment-trap moorings offCape Blanc, Mauritania and compared them to observed wind-speed and precipitation as well as satelliteimages. Our results indicate a clear seasonality in the grain-size distributions: during summer the modalgrain sizes were generally larger and the sorting was generally less pronounced compared to the winterseason.Gravitational settling was the major deposition process during winter. We conclude that the followingtwo mechanisms control the modal grain size of the collected dust during summer: (1) wet depositioncauses increased deposition fluxes resulting in coarser modal grain sizes and (2) the development of coldfronts favors the emission and transport of coarse particles off Cape Blanc. Individual dust-storm eventsthroughout the year could be recognized in the traps as anomalously coarse-grained samples. Duringwinter and spring, intense cyclonic dust-storm events in the dust-source region explained the enhancedemission and transport of a larger component of coarse particles off Cape Blanc. The outcome of our studyprovides important implications for climate modellers and paleo-climatologists.? 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license