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Transatlantic fluxes of Saharan dust: changing climate through fertilising the ocean?

Final Report Summary - DUSTTRAFFIC (Transatlantic fluxes of Saharan dust: changing climate through fertilising the ocean?)

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. 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. In DUSTTRAFFIC, we had the unique opportunity to study transport and deposition of Saharan dust, as well as its marine environmental effects, using so-called marine sediment traps as well as dust-collecting buoys that were deployed along a transatlantic transect at 12oN, directly below the core of the Saharan dust plume that blows across the Atlantic.
The project shed new lights on downwind transatlantic trends in transport and deposition of Saharan dust through time; strong gradients exist from East to West, which is intuitively logical. It was observed that the ballasting potential of Saharan dust particles on the eastern side of the Atlantic is largest, although this ballasting effect can reach a maximum, after which marine-snow particles do not take up additional dust particles any longer (cf. Van der Jagt et al., L&O accepted).
Moreover, downwind sorting of dust particles leads to strong gradients in both the particle size of wind-blown dust (Van der Does et al., 2016, ACP) as well as their depositional fluxes (Korte et al., 2017, ACP), which both are larger in summer compared to winter. The seasonal contrast diminishes in amplitude downwind from East to West.
In terms of composition, there are also large gradients from East to West with large and heavy quartz particles making the larger part of the dust load close to the African coast as opposed to platy- and clay minerals on the Caribbean side (Korte et al.. 2017, ACP).
One of the novel outcomes of the project is that most of the dust deposited in the Atlantic Ocean is washed out of the atmosphere by summer rains. These field data are corroborated by comprehensive earth system model (CESM) dust-deposition data across the same transatlantic transect. In addition, 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– (phosphates, silicates) and micronutrients (iron) 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 otherwise limited by macro– or micronutrients (Van der Does et al., submitted).
Micropaleontological studies demonstrate that there are indeed conditions at which opportunistic phytoplankton species bloom as a result of Saharan dust deposition (Guerreiro et al., 2017, BG).