Saharan dust at the source

Publication date: Wednesday 04 July 2018

Category: DUST

In their new paper, Michèlle van der Does and colleagues show how the so-called radiogenic isotopes of Sr, Nd, and Hf, as well as a suite of rare-earth elements can be used to demonstrate the Saharan origin of sediments collected while sinking towards the seafloor of the Atlantic Ocean. These particles were collected using an array of instruments that were moored along a transect between Africa and the Caribbean. Comparison of sediments collected from these marine sediment traps, from the atmosphere during ship cruises, from various places in deserts in Mauritania, with dust samples collected on Barbados, and with published literature on Amazon-River samples clearly shows the Saharan origin of the samples collected at sea. (should anyone have doubted it....)

Saharan dust at the source: Jraif, Mauritania

One of the many different Saharan dust sources in Mauritania; streamers of dust are blown off and perpendicular to the sand ripples

Each year, about 180 million tons of dust are blown westward from the northwest African coast (Yu et al., 2015). Using an array of moored sediment traps anchored at 4,000-6,000 meter on the seafloor of the Atlantic Ocean, we study the spatial and temporal variability of dust that is deposited in the ocean. In addition, we sample the atmosphere during ship cruises with dust collectors standing on deck. The shipboard dust collections are snapshots in time but with the moored sediment traps and dust-collecting buoys we are establishing time series of dust transport and deposition.

In addition to Saharan dust, there are also large amounts of sediments flown into the equatorial Atlantic Ocean through discharge of South American rivers like the Amazon and Orinoco. Potentially, these sediments can also reach the sediment traps along our transect of instruments between Africa and the Caribbean.

There are several ways to characterize these different sediments and a much-used approach is by studying the chemical composition of the material of interest and then compare this to well-known composition of various potential source areas. Michèlle went to the Rosenstiel School for Marine and Atmospheric Sciences in Miami, USA to work with Ali Pourmand and Arash Sharifi and analyze the radiogenic isotopes of strontium (Sr), neodymium (Nd) and hafnium (Hf) and a suite of rare-earth elements (REEs). By comparing the chemical composition of a suite of samples collected along a source-to-sink transect starting on the African continent and ending on Barbados, with published compositions of Amazon-River samples, Michèlle and colleagues managed to undisputably demonstrate the Saharan origin of samples collected by the marine sediment traps.

Saharan dust at the source: Akjoujt, Mauritania

During a field trip to Mauritania in 2009, a number of potential sources of Saharan dust were sampled. We can now compare these samples to those collected at sea.

Publication details: Michèlle van der Does, Ali Pourmand, Arash Sharifi & Jan-Berend Stuut (2018) North African mineral dust across the tropical Atlantic Ocean: insights from dust particle size, radiogenic Sr-Nd-Hf isotopes and rare earth elements (REE) Aeolian Research 33, p 106-116 https://doi.org/10.1016/j.aeolia.2018.06.001

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Dust

Why are we so interested in dust?

It turns out that there are many direct and indirect links between dust and climate. The most straightforward one is fine-grained dust in the upper atmosphere blocking incoming sunlight, causing a net cooling effect. But there are warming effects too; in the lower atmosphere, coarser-grained dust particles absorb energy that was refelected at the earth's surface and this way, dust acts as a greenhouse gas.

There are many more negative and positive climate-related effects but the main link to the ocean is the fact that marine life may profit from nutrients in dust. When plankton reproduces, it takes up CO2 from the atmosphere. Thus, dust could potentially act as an ocean fertilizer, sequestering a greenhouse gas!

Recently published papers

Van der Does, M.; Pourmand, A.; Sharifi, A.; Stuut, J-B W. 2018. North African mineral dust across the tropical Atlantic Ocean: insights from dust particle size, radiogenic Sr-Nd-Hf isotopes and rare earth elements (REE). Aeolian Research 33, 106-116 doi:10.1016/j.aeolia.2018.06.001
Van der Jagt, H.; Friese, C.; Stuut, J-B W.; Fischer, G. & Iversen, M.H. (2018). The ballasting effect of Saharan dust deposition on aggregate dynamics and carbon export: Aggregation, settling, and scavenging potential of marine snow. Limnology and Oceanography 63, 1386-1394, doi:10.1002/lno.10779 .
Schreuder, L.T., Stuut, J-B W., Korte, L.F., Sinninghe-Damsté, J.S., Schouten, S. 2018.
Aeolian transport and deposition of plant wax n-alkanes across the tropical North Atlantic Ocean. Organic Geochemistry 115, 113-123
Guerreiro, C.V., Baumann, K.H., Brummer, G.J.A., Fischer, G., Korte, L.F., Merkel, U., Sá, C., de Stigter, H., Stuut, J.B.W., 2017. Coccolithophore fluxes in the open tropical North Atlantic: influence of the Amazon river and of Saharan dust deposition.Biogeosciences Discuss. 2017, 1-26.
Korte, L.F., Brummer, G.J.A., van der Does, M., Guerreiro, C.V., Hennekam, R., van Hateren, J.A., Jong, D., Munday, C.I., Schouten, S., Stuut, J.B.W., 2017. Downward particle fluxes of biogenic matter and Saharan dust across the equatorial North Atlantic. Atmos. Chem. Phys. 17, 6023-6040.
Van der Does, M., Korte, L.F., Munday, C.I., Brummer, G.J.A., Stuut, J.B.W., 2016. Particle size traces modern Saharan dust transport and deposition across the equatorial North Atlantic. Atmos. Chem. Phys. 16, 13697-13710.

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Saharan dust at the source

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