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It is notable that over the period to most SPA annual means fall within a narrow range and show no evidence of a trend.

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However, after. Understanding dust and climate While rainfall is a critically important factor for dust emission, we know that other factors also play a role Okin et al. In the future, land use and land disturbance will become increasingly important Ginoux et al. The complexity of these relationships presents a challenge to the modeling community Huneeus et al. Figure 2: The annual cycles of monthly mean dust concentrations at University of Miami network sites in the North Atlantic. The means at each site are based on multi-year data but they are not necessarily measured concurrently.

Note the difference in the ordinate scales in each inset figure.

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Figure adapted from Prospero et al. Although dust concentrations are low in winter and spring on Barbados, we know that there is intense dust activity in North Africa in the Sahel region, most prominently in the Bodele Depression in northern Chad Ben-Ami et al. Bodele sediments, deposited during the Holocene in what was Lake Mega Chad, are now being mobilized in huge quantities Washington et al. Almost daily in spring, satellites show immense plumes being carried westward across the Atlantic, many eventually reaching South America in the region of French Guiana, km from the source.

Ten years of recent air-quality aerosol measurements from Cayenne, a coastal town in French Guiana The magnitude and persistence of dust transport has had a significant impact on the supply of minerals and nutrients to the soils and sediments to the Caribbean and South America including the Amazon; Ben-Ami et al.

Dust concentrations at all sites except Mace Head Ireland and Storhofdi Iceland are dominated by African dust, most notably in the subtropical and tropical regions. Iceland dust is transported by winds from paraglacial deposits in southeast Iceland and deposited to the North Atlantic where it might serve as an important source of nutrient iron Prospero et al. The summer maximum observed at Barbados, Miami, and Bermuda contrasts greatly with the spring maximum at Cayenne, which reflects the combined effects of the seasonal shifts in African dust sources and in the transporting wind systems.

This contrast shows that dust transport processes to the eastern Atlantic differ from those that carry dust to the western Atlantic Stuut et al. Africa, as the world's largest present-day dust source, presents a special challenge when anticipating the changes in dust emissions in coming decades.

It is notable that in the recent IPCC assessment, models forecast that large areas of North Africa will become drier Seneviratne et al. However, the models could not agree on the future trends in large areas of North Africa that today are the most active dust sources, most notably those in the Sahara and the Sahel. Thus, it is important that we continue to monitor transport from Africa to the Atlantic in a systematic manner so as to track changes over this region and to test the ability of dust-climate models to characterize long-range transport to the oceans Schulz et al.

In order to more fully understand the various factors affecting dust mobilization we need a better understanding of the most active source areas Prospero et al. This knowledge is important to interpreting dust-climate. The significance of particle size of long-range transported mineral dust Jan-Berend W. Stuut1,2 and Maarten A.

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The physical properties of mineral-dust particles turn out to be more important than hitherto recognized; their composition, radiative properties, and fertilizing and ballasting potential fluctuate on various spatial and temporal scales. Especially the impact of large dust particles is often underestimated. The role of aeolian dust for the radiation balance of the Earth is size-dependent.

Claquin et al. Otto et al. Therefore, to account for the climatic effect of mineral dust it is critical to have a good understanding of the size distribution of suspended particles in space and time. For this reason, these large dust particles are often simply not measured.

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As a consequence,. Grain size sorting during transport The particle-size distribution of mineral dust is not only a function of lateral but also vertical transport distance e. This observation had already been put into a conceptual model by Pye and Zhou ; Fig. As wind is a very size-selective transport mechanism, the sediments it had carried typically a well-sorted particle-size distribution, which gradually gets finer from proximal to distal deposition sites. This fact has been used in numerous paleo-environmental studies to both determine source-to-sink changes in the particle size of aeolian dust e.

Sarnthein et al. Prins and Weltje ; Moreno et al. Obviously, at proximal locations where aeolian dust is deposited relatively close to its source , particles are coarser grained and mass accumulation rates are larger than at more distal locations where aeolian dust is deposited relatively far from its source. This observation was first.

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Evidence for large aeolian particles Despite the availability of these conceptual models and many grain-size related studies, there are still many unresolved questions related to the physical properties of wind-blown particles, particularly their particle size and shape.

Middleton et al. This particle-size constraint can be explained by the physical laws of entrainment and settling velocities e. Gillette However, various aeolian-dust records from loess and marine sediments contain considerable amounts of coarse-silt and fine-sand grains, i. For example, quartz particles originating from the Asian. Figure 1: Schematic diagram showing different dust-transport mechanisms in the high- and low-level atmosphere redrawn from Pye and Zhou From the above, we infer that the largest ballasting and scavenging effect will take place at proximal deposition sites.

Thus, even without adding nutrients to the ocean, mineral dust may increase the C-cycle by speeding up the export of organic material to the sea floor. On the other hand, particle size and associated sinking speed also impacts the bio-availability of the dust-borne micro-nutrients; the faster the particles sink, the less time remains to mobilize the nutrients they carry.

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Therefore, at distal locations, where dust particles are small and the size-dependent specific surface area of the particles is high, the amount of nutrients per mass is much higher than at proximal sites. We speculate that there is an ideal source-to-sink distance where dust particles are large enough to have an optimal ballasting effect, but also small enough to have an optimal fertilizing effect.

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Figure 2: Map of the subtropical northeastern Atlantic, showing the decrease in both particle size and dust flux as measured in surface sediments by Sarnthein et al. New unpublished results from transatlantic Saharan dust-monitoring projects e. These observations suggest that the assumptions regarding the transport capacity of individual dust outbreaks need to be revised e. Such revisions would have consequences not only for the interpretation of the proxy records, but also for the quantification of the climate-forcing effect of wind-blown particles in the atmosphere e.

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Marine production and export The size and shape of aeolian particles and dust-transport processes may also have affected past atmospheric CO2 changes, due to the role that sinking dust particles may play in the biological carbon pump in the oceans. The increased primary productivity and its downward export sequestered additional CO2 from the atmosphere see also Martinez-Garcia and Winckler, this issue. A crucial step in this process is the export of organic matter from the surface ocean to the sea floor.

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Many Fefertilization experiments were carried out as a consequence of the postulated Fe hypothesis e. They demonstrated that it is indeed possible to boost marine life with artificially added dissolved micro-nutrients, but that most of the generated organic tissue containing the sequestered CO2 is being recycled in the surface ocean layer. Here, the dust particles may play a crucial role by providing mineral ballast for the aggregation and sinking of the organic tissue. If this ballasting process is fast enough, there is less time for the organic tissue to be recycled and the export of organic matter becomes more efficient.

The speed of the downward transport from surface to the deep ocean is directly related to the size of the dust particles that act as anchors for organic matter. Intuitively, one would think there is an upper limit to size and potential scavenging; a brick would be too large and sink too fast to have a significant scavenging effect. Outlook The aforementioned role of mineral-dust particles assumed all deposition to be the result of gravitational settling from the atmosphere.

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However, hardly anything is known about wet deposition of mineral dust. New observations Prospero, pers. Transatlantic observation projects have been set up to shed light at source-to-sink variability at high temporal resolution. Next to the physical and chemical properties, also the temporal and spatial micro- biological effects on nutrient availability and scavenging need to be studied, as they may play a dominant role on the aforementioned physical processes. Mineralogical, elemental and isotopic measurements point to Eastern Asia as the main source of dust reaching Greenland, both in the present and during the last glacial period.

Data suggest that the bulk of the dust derives from the Taklamakan Desert of western China. Determining the provenance of ice core dust is key to our understanding of the prominent variability in the dust concentration observed in the Greenland and Antarctic ice core records see also Vallelonga et al. Greenland dust provenance was a matter of speculation until the mid 90s.

Some studies based on modern aerosol elemental composition suggested the Saharan region as a possible source Mosher et al. Based mainly on the clay mineralogy of the dust, Biscaye et al. Locations of the main deserts. Taklamakan and Northern China deserts are indicated together with the main winds dotted lines and dust transport pathways solid lines in the spring adapted from Sun et al.

They showed that instead it closely resembled the fine material deposited in the Chinese Loess Plateau region and must therefore have derived from adjacent deserts. An Eastern Asian provenance was also supported by radiogenic isotope ratios Sr, Nd and Pb , which were consistent with a Chinese Loess Plateau signature, taking into account the fact that ice core samples unavoidably contain variable amounts of volcanogenic particles mixed with the dust.

Little change over time Only minor variability was observed in the mineralogical and isotopic characteristics of these ice core dust samples Biscaye et al. Also, the mineralogical and isotopic characteristics of dust extracted from Holocene ice core sections at various elevated sites across central Greenland did not depart significantly from glacial dust signatures Svensson ; Bory et al.

So, overall, the mineralogical and isotopic signatures of dust in icecores all point to Eastern Asia as the overriding supplier of the dust to the top of the Greenland ice cap, both under glacial and inter-glacial climate conditions. Clues from snow deposits in Greenland In order to gain further insights into the contributing source region s , especially.