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Mineral dust aerosol constitutes an important component of the Earth’s climate system, not only on short timescales due to direct and indirect influences on the radiation budget, but also on long timescales by acting as a fertilizer for the biosphere and thus affecting the global carbon cycle. For a quantitative assessment of its impact on the global climate, state-of-the-art atmospheric and aerosol models can be utilized. In this study, we use the ECHAM6.3-HAM2.3 model to perform global simulations of the mineral dust cycle for present-day (PD), pre-industrial (PI) and Last Glacial Maximum (LGM) climate conditions. The intercomparison with marine sediment and ice core data as well as other modeling studies shows that the obtained annual dust emissions of 1221, 923 and 5159 Tg for PD, PI and LGM, respectively, generally agree well with previous findings. Our analyses focussing on the Southern Hemisphere suggest that over 90 % of the mineral dust deposited over Antarctica are of Australian or South American origin during both PI and LGM. However, contrary to previous studies, we find that Australia contributes a higher proportion during the LGM, which is mainly caused by changes in the precipitation patterns. Obtained increased particle radii during the LGM can be traced back to increased sulphate condensation on the particle surfaces as a consequence of longer particle lifetimes. The meridional transport of mineral dust from its source regions to the South Pole takes place at different altitudes, depending on the grain size of the dust particles. We find a trend of generally lower transport heights during the LGM compared to PI as a consequence of reduced convection due to colder surfaces, indicating a vertically less extensive Polar Cell.

© 2021. The Authors.Saharan dust is transported in great quantities from the North African continent every year, most of which is deposited across the North Atlantic Ocean. This dust impacts regional and global climate by affecting the atmospheric radiation balance and altering ocean carbon budgets. However, little research has been carried out on time series of Saharan dust collected in situ across the open Atlantic. Here, we present a unique three-year time series of Saharan dust along a trans-Atlantic transect, sampled by moored surface buoys and subsurface sediment traps. Results show a good correlation between the particle-size distributions of atmospheric dust and the lithogenic particles settling to the deep ocean floor, confirming the aeolian origin of the lithogenic particles intercepted by the subsurface sediment traps, even in the distal western part of the Atlantic Ocean. Dust from both dry and wet deposition as collected by the sediment traps, shows increased deposition fluxes and coarser grain size in summer and/or autumn that coincides with increased precipitation at the sampling sites as derived from satellite data. In contrast, both buoys that sampled dust during transport at sea level show little seasonal variation in both concentration and particle size, as the large amounts of dust transported in summer and early autumn at high altitudes are far above their sampling range. This implies that wet deposition in summer and autumn defines the typical seasonal trends of both the dust deposition flux and its particle-size distribution observed in the sediment traps.

Mineral dust plays an important role in the ocean’s carbon cycle through the input of nutrients and metals which potentially fertilise phytoplankton, and by ballasting organic matter from the surface ocean to the deep sea floor. In addition, mineral dust feeds back on climate through many different pathways like changing the radiative balance of the atmosphere, by stimulating cloud formation, dampening hurricane formation and by changing the earth’s albedo. Because open-ocean dust-flux measurements are either based on shipboard- or sediment-trap data, they are biased by interpolation and extrapolation of point observations in space and time. Alternatively, dust-flux estimations can be made using satellite observations, but these are often hampered by the presence of clouds. For these reasons we have been studying Saharan dust along a Transatlantic transect between northwest Africa and the Caribbean, focussing on temporal and spatial variability of dust-deposition fluxes and how these are reflected in terms of dust particle size and composition. One important finding deals with the deposition of Saharan dust by rain, which seems to be an important way to make nutrients available to phytoplankton living in the surface ocean. Nutrient-release bottle experiments in ambient sea water carried out along the same transect demonstrate how wet deposition of Saharan dust increases the release of both macro- (P, Si) and micronutrients (Fe) up to an order-of-magnitude as opposed to dry deposition. Rain-amplified bioavailability of these nutrients may well be the key to increased surface-ocean productivity in the remote and oligotrophic parts of the oceans and, potentially, also continental ecosystems. See: www.nioz.nl/dust

Mineral dust plays an important role in the atmospheric radiation budget as well as in the ocean carbon cycle through fertilization and by ballasting of settling organic matter. However, observational records of open-ocean dust deposition are sparse. Here, we present the spatial and temporal evolution of Saharan dust deposition over 2 years from marine sediment traps across the North Atlantic, directly below the core of the Saharan dust plume, with highest dust fluxes observed in summer. We combined the observed deposition fluxes with model simulations and satellite observations and argue that dust deposition in the Atlantic is predominantly controlled by summer rains. The dominant depositional pathway changes from wet deposition in summer to dry deposition in winter. Wet deposition has previously been suggested to increase the release of dust-derived nutrients and their bioavailability, which may be a key contributor to surface-ocean productivity in remote and oligotrophic parts of the oceans.

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

Massive amounts of Saharan dust are blown from the African coast across the Atlantic Ocean towards the Americas each year. This dust has, depending on its chemistry, direct and indirect effects on global climate including 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 northwest Africa and the Caribbean along 12⁰ N, in a down-wind extension of the land-based dust collectors placed at 19⁰ N on the Mauritanian coast in Iwik. 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 synchronised 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 down-wind 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 Iwik. Down-wind 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, 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 Saharan dust transport and deposition from source to sink and its impact on fertilization and carbon export to the deep ocean.