MULTIPLE DUST SOURCES IN THE SAHARA:
IMPLICATIONS FOR MODELING DUST EMISSION AND TRANSPORT
Onn Crouvi 1,2*, Kerstin Schepanski 3, Rivka Amit 1, Alan R. Gillespie 4, Yehouda Enzel 2
1
Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel; *presenting author: [email protected]; 2 The Fredy and Nadine Herrmann Institute of
Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel; 3 Leibniz Institute for Tropospheric Research (TROPOS), Permoser Str. 15, 04318
Leipzig, Germany; 4 Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA
What is the potential contribution of each map unit as an active dust sources?
Background
Mineral dust plays multiple roles in mediating physical and biogeochemical exchanges among the atmosphere, land and
ocean, and thus is an active component of the global climate system. In order to estimate the past, current and future
impacts of dust on the climate, sources of dust and their erodibility should be identified.
The Sahara is the major source of dust on Earth. Based on qualitative analysis of remotely sensed data with low
temporal resolution, the main sources of dust that have been identified are topographic depressions comprising of dry
lake deposits and playas in hypr-arid regions (Prospero et al., 2002). Yet, recent studies cast doubts on these as the
major sources and call for a search for others. Moreover, the susceptibility of different soils to eoilian erosion (wind land
erodibility) in the Sahara is still poorly known.
Goals
1. To identify current sources of atmospheric dust in the Sahara in term of soil types and geomorphic units.
2. To estimate land erodibility of soil types and geomorphic units in the Sahara.
Methodology
1. Identification of dust sources - Quantitative spatial correlation (in GIS) between number of days with dust storms (2yr data, 2006-2008) and soil type/geomorphic unit, in a 1°×1° grid cell.
2. Estimation of soil types/geomorphic units land erodibility - Analysis of the covariance (ANCOVA) between 1) number
of days with dust storms, 2) number of days with high-wind speed events, and 3) soil types/geomorphic units.
We calculated the potential contribution of each map unit by calculating its average NDS:
Several units that are currently not an
important dust sources (low NDS), are
potentially important dust sources (high
avg. NDS):
• Rock debris
• Fluvisols
• Luvisols
• Gypsisols
This discrepancy can be explained by the
limited distribution of these map units
over windy regions in the Sahara.
NDSi
Avg.NDSi 
ni
Map unit
Average NDS
Sand dunes
21.8
Rock debris
20.4
Fluvisols
11.7
Map unit
Average NDS
Leptosols
11.6
Gleysols
6.7
Cambisols
10.5
Regosols
4.4
Arenosols
9.9
Solonetz
4.3
Calcisols
9.5
Solonchaks
2.4
Luvisols
8.4
Planosols
1.5
Gypsisols
7.1
Vertisols
1.0
Dustiest place on Earth (The Bodélé depression) emits not only diatoms !
Data
Soil types and geomorphic units
We used the Harmonized World
Soil Database (FAO). Soil types are
classified
partly
by
their
composition and geomorphologic
context,
depending
on
the
classification scheme.
Barchan sand dunes
composed of quartz
Vermeesch and Drake (2008)
We chose the dominant soil for
each grid cell using two different
approaches:
“Objective” - Map unit with highest
areal extent
“Playa biased” - As above, but map
units that characterize playa/dry
lake beds are selected if present,
regardless of their areal extent
Diatomite – soft
rock composed of
diatoms
Bristow et al. (2009)
The land erodibility of the Sahara’s soil types and geomorphic units
• The results of the ANCOVA show that each of the two examined covariates (soil data and ,NWE) and their
interaction, control the NDS (P < 0.001).
• Ranking map units according to their regression slope can be interpreted as ranking the erodibility of these units:
high slope values are attributed to sand dunes (0.53) and gypsisols (0.45), and intermediate slope values were found
for leptosols (0.23), rock debris (0.20), calcisols (0.18) and arenosols (0.14). Units with the lowest slopes are vertisols
(0.08), and lixisols (0.03).
Number of days with dust storms
(NDS)
Location of the origin of the dust
storms was identified using high
temporal resolution remotely
sensed images (SEVIRI; Schepanski
et al., 2009) , and counted per grid
cell of 1°×1° for the 2-yr data.
These results are in
general agreement with
field and laboratory
experiments for
estimating soil erodibility
of different soil types and
soil textures.
NDS were grouped per soil
type/geomorphic unit:
ni
NDS i   NDS i , j
j 1
Number of days with wind events
(NWE)
Nocturnal low-level jets (LLJ) are
the dominant mechanism for dust
emission
in
the
Sahara
(Schepanski et al., 2009).
Differences in wind speed at 900
hPa and 750 hPa levels was used
as a proxy for LLJ occurrence. If
the difference > 6 m s-1, a highspeed wind event was defined for
this day. Winds events were
summed per pixel yielding NWE.
Implications to current, past, and future dust emission models
Current dust emission models (e.g., Laurent et al., 2008) use sandblasting as the sole mechanism of dust emission;
these models underestimate dust emission in southern Sahara, area rich with sand dunes and arenosols. We propose
that the fracturing of saltating sand and the removal of clay coatings from sand grains through eolian abrasion is the
dominant dust-emission mechanism for the vast sand-rich areas of the Sahara. This process has been proven to
produce fine dust from sand grains in experimental studies (Bullard et al., 2004) and is the most probable explanation
for the formation of coarse silts from active dune fields (Crouvi et al., 2010).
Which soil types/geomorphic units are the most frequent active dust sources?
We calculated the NDS per soil type/geomorphic unit out of total NDS to
determine the relative importance of each map unit in producing dust storms:
Using the “objective” approach for choosing the
dominant map unit, >90% of the total dust storms
originated from :
• Sand dunes
• Leptosols (thin, gravel-rich soils)
• Calcisols (e.g., alluvial fans)
• Arenosols (stabilized sand dunes)
• Rock debris
Playa soils explain less than 1%
NDS
NDS i (%)  1502 i 100
 NDS
j 1
Our results can explain the past increased dustiness
during the last glacial period (2-4 times higher than
during the Holocene), when sand dunes are thought to
have been more common than during the Holocene.
The observations of concurrent high accumulation rates
of dust in Atlantic Ocean cores off northwest Africa and
of continental loess sequences along the edges of the
Sahara, downwind of the sand fields (Crouvi et al.,
2010), support our hypothesis.
Dust emission over the Sahara is expected to increase
under projected future conditions. Using the linear
regressions presented in this study we can predict the
increase in NDS for a given increment of NWE. For
example, if an increase of NWE by 125 days will occur,
sand dunes are predicted to produce on average
additional 24 NDS, versus vertisols that will add on
average only 0.5 NDS.
Crouvi et al. (2010)
We suggest that considering aeolian abrasion of sand grains in dust-emission and
transport models will improve model results for past, current and future scenarios.
References:
Using the “playa biased” approach for choosing the
dominant map unit, playa and dry lake beds soils are
responsible for maximum 18% of the total NDS data.
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