Sahara air incursions and dust deposition over the Canary Islands and its impact on the
terrestrial and oceanic environment
Dorta-Antequera, P.*; Gelado-Caballero, Mª.D.**; Criado-Hernández, C*. M.D.**; Cardona-Castellano, P.**; ColladoSánchez, C.**; Hernández-Brito, J.J.**; Mendoza-Rodríguez, S.*; Torres-Padrón, M.E.**; Rodríguez-Somoza, M.J.**
and Siruela-Matos, V.F.**
*Department of Geography. University of La Laguna.
**Department of Chemistry. University of Las Palmas de Gran Canaria
Abstract
The Sahara, the largest desert in the world is a source of dry hot air and also dust. Its
position, very close to the Canary archipelago has a hard influence on the climate of this area
because of the inexhaustible supply of dust and the high temperatures have adverse effects on the
environments and socioeconomic aspects such as health, forest fires frequency, crops. This paper
examines this topic quantitatively.
Key words
Sahara Dust, Tropical Continental Air Mass, Canary Islands
INTRODUCCION
This study is a part of a research project based in techniques from Synoptic Climatology and
multivariate statistics, used in previous research (DORTA, 1999 y DORTA et al., 2003), together
with data from the measurement of dust deposits and chemical composition.
To get the results we have worked with a base data meteorological and synoptic and
samples of dust collected in the summit of Gran Canaria Island using high volume traps to
understand the phenomena from a historical perspective and today. In addition we have studied
some samples from cross-sections located inside of Quaternary sand-dunes to understand the dust
invasion over a long term periods.
The scientific target are to know if the phenomenon, is increasing currently and aims if so,
there is a relation the Global Climate Change for this We are analyse several variables: frequency,
intensity, volume, geographical source of dust, etc). More accurate information about the influence
of Sahara dust in the geomorphological evolution and soil features and also on the ocean around the
islands.
SAHARA AIR AND ENVIRONMENT
The climate of the Canary Islands is under the influence of three different Air Masses
(MARZOL, 1993): Tropical Oceanic, Tropical Continental and Polar Oceanic; the most frequent is
the first, mainly during the summer, while in winter the frequency of the others increases, to be
about half winter days. The Sahara Air Mass enters the atmosphere mainly in inter (more than 30%
of days) and with its minimal frequency in spring (DORTA, 1999).
The air from the Sahara exhibits thermic and hygrometric pattern, already studied by other
authors (FONT TULLOT, 1950; DORTA, 1990, 1991 and 1999), also its effects (DORTA, 1995
and 2001; GARCÍA et al., 2001) has even been analyzed it frequency and seasonality using time
series with different longitudes (DORTA, 1999 and 2003). This air mass can be easily defined by
the increase in temperature – although in winter normally it is not so great -, a decreased in
hygrometric values, wind mainly from the East, stable atmosphere and impaired visibility due to
airborne dust.
Its arrival has negative impacts on the population and agriculture; there is evidence of
increased atmospheric pollution because the inversion level is lower than normal Trade-wind and,
on the other hand, the dryness and high temperature has negative effects on crops.
The risk of forest fires becomes greater, mainly at the altitudinal level occupied by forest of
Pinus canariensis (figure 1). Indeed the most part of surface burnt has in coincided with Sahara air
invasion (DORTA, 2001).
Figure 1: Differences in temperature, wind speed, and humidity in the three air masses at different
isobaric surfaces (Data derived from thermodynamic coring in Santa Cruz de Tenerife).
Temperatures (°C)
Humidity (%)
80
70
60
50
40
30
20
10
0
30
25
20
15
10
5
0
Maritime
air
Sea level
A 850 hPa
Saharan
air
A 925 hPa
Wind (m/s)
16
14
12
10
8
6
4
2
0
Maritime
air
Sea level
A 850 hPa
Saharan
air
A 925 hPa
Maritime
air
Sea level
A 850 hPa
Saharan
air
A 925 hPa
Own elaboration
Saharan Dust and Environment
We recently began to study the dust from a different point of view (CRIADO and DORTA,
2003; DORTA et al., 2001; QUEROL et al., 2002; TORRES-PADRON et al., 2003 and GELADO
et al., 2003).
The Sahara desert normally has dirty atmosphere due to the presence of dust particles, but
the transport of massive amounts of this kind of material occurs in dust storms. After the aspiration
of dust up into the storm the movement of the dust plume depends on the position of pressure
centres; the transport can reach very distant areas, even the Caribean Sea and Amazon basin
(ARIMOTO, 2001), a long-term suspension may continue for a week.
Deposition on the Canary soils could has been important over a geological time scale. There
are several papers about soils of the Canary Island citing the presence of allochtonous minerals in
their composition (FERNANDEZ CALDAS et al., 1982). The most abundant is quartz, not present
in the volcanic geology of the archipelago (except in the trachytic outcrop of Montaña de Tindaya,
Fuerteventura); its presence in soils has been explained as a result of Sahara dust invasion. A recent
research (MIZOTA and MATSUHISA, 1995) present evidences for a Sahara source of quartz
through of analysis of abundance of δ18 O. Another mineral present in the Canary soils is mica; illite
is very abundant in the argilic soils of Fuerteventura and Lanzarote (TEJEDOR et al., 1995;
TORRES, 1995); At present there is not clear relationships between the basaltic composition of
these two islands, explained by weathering on airborne micas from the Sahara; the analysis of
ratios 87Sr/86Sr show values very close to those for airborne micas actually from Sahara desert
(MIZOTA and MATSUHISA, 1995). Finally, in several mineralogical analyses using XRD we
have found calcite issued from the Mesozoic and Neogene calcareous outcrops of Western Sahara
and Hammada of Tindouf. Perhaps this supply of carbonates could be related with the calcretes
development in the Eastern Canary Islands.
From a geomorphological point of view there is not evidence for peridesertic loess, very
important, for instance, on the northern edge of the Sahara. The absence of this kind of Quaternary
deposit in the archipelago can be related with the position of the Canary Island to the west of the
Sahara; so, the heaviest dust deposition, normally linked with heavy rains, are produce by a low
pressure through, moving across the Sahara, and passing over the archipelago A dense dust cloud
can lead to a massive deposition as Blood Rain. This situation is not very common but we have
identified recent examples (CRIADO and DORTA, 2003).
Ancients massive dust deposition was studied in three cross-sections of littoral sand-dunes
in Fuerteventura and Gran Canaria. We have found evidence for at least four periods with
stabilization of sand-dunes between 30 ka and 9.5 ka ago (CRIADO and HANSEN, 2000).
Figure 2: Combined profiles of the cross-sections of Corralejo, Jinamar and Tufía
Own elaboration
The first one, called Corralejo cross-section (Fuerteventura island), presents 4 layers: C.2.1
and C.2.3 are composed of biogenic sand, very rich in carbonate (between 84 - 89%) including
aragonite, calcite and magnesium calcite and very little aeolian quartz from Sahara. They are
ancient coastal dunes, actives coinciding with an arid period. The layers C.2.2 and C.2.4, Munsell
colour 10 YR 7/4 and containing 22.8% and 17.0% of fraction <63 µm in each sample, show
abundant quartz and bee nets (Antophora sp.) and shells of terrestrial snails (Teba sp.) allowing a
14
C dating in 16980±120 BP (Beta-125056) and 13890±110 BP (Beta-125057). Analysis revealed a
mixture of sand of coastal dune and Sahara dust. It would be responsible for stabilization of sanddunes during palaeoclimatic conditions rainier than today, producing a massive deposition of dust
as Blood Rain and doing also a phytostabilization.
The second one, Tufia (Gran Canaria), presents three layers. T.1.2A is a biogenic sand layer
and biogenic: aragonite, calcite and magnesium calcite. T.1.2B is richest in silt (14.7% <63 µm)
with shells (Pomatias laevigata) and little quartz in the fine fraction; it has been dated at 30990±300
BP (Gd-7560). On top of this cross-section we can see an ancient sand-dune very rich in volcanic
grains.
The third one, Jinamar (Gran Canaria), present in the lowest layer an old sand-dune rich in
carbonates -aragonite, calcite and calcite magnesium- with a good sorting. Above it there are two
layers with a silty appearance (64.9% and 62.8% < 63 µm), 7.5 YR 5/4 Munsell colour, abundant
Aeolian quartz and rich in bee nets (Antophora) and terrestrial snails (Teba pisana, Hemycicla
glaciana, Napeus obesatus, Turricula despreaxi y Pomatias laevigata). It has been dated at
9530±110 BP (Beta-60644).
So, during the last 30 ka there were at least four periods with stabilization of coastal sanddunes, associated with massive deposition of aeolian Sahara quartz in coincidence with heavy rains.
Normally today, dust invasions occurs mainly in a stable atmosphere; however several dust storms,
with massive deposition of fine fraction as Blood Rain involving heavy rains has been noted
(CRIADO y DORTA, 2003). During these four periods this kind of weather may well have been
more frequent than today.
The influence of arrival of Sahara dust in the sea has notable effects as we can deduce from
the recent publications on this topic (TORRES-PADRON, 2002). The total amount falling on the
ocean surface is greater than on the continents; this supply of mineral material can affect the
biogeochemical cycles of trace elements and nutrients, modifying the productivity of marine
ecosystems. However, we must note that the relationships are very complex due to physical
processes in the surface waters and processes within in the biogeochemical cycles. There are now
numerous quantifications of dust airborne over the Atlantic Ocean (PROSPERO and CARLSON,
1972 and 1981). The Sahara desert is the main source of dust raising the atmosphere (200 x 106
tons). Today the feature most interesting feature in this field is the importance of iron as nutrient
trace in the biogeochemistry of surface waters. In several oceanic areas the low levels of iron limit
the growth of phytoplankton, even if there are sufficient quantities of other nutrients. In addition,
important iron supplies probably could stimulate nitrogen fixation in waters of North Atlantic
circulation, stimulating the growing of phytoplankton communities (GRUBER and SARMIENTO,
1997). We must note that the airborne material from Sahara desert also contain biological species
(bacteria, for instance) and they can modify the ecological equilibrium in the coral reefs. At last, we
must note the current controversy about the role played by aerosols in the Climatic Change.
For the Canary oceanic area the measurements show values 1 to 2.4 x 106 tons (TORRESPADRON et al., 2002). Recent studies made by the Department of Chemistry of University of Las
Palmas, in Gran Canaria, show the importance of these supplies in the terrestrial and oceanic
environments. They show a clear seasonal pattern and a wide annual variability, with a maximum in
frequency and amount in winter and a minimum in spring. The mean annual concentrations are
between 27 to 70 µg.m-3, with a peak above 500 µg.m-3 , with constant wind from the Sahara and
the mean grain-size 0.6 to 4.9 µm (GELADO et al., 2003).
CONCLUSIONS
The main aim of this paper is to confirm the importance of Sahara dust incursions in the
climate of the Canary Islands by its frequency and, especially by its effects. The environment –
terrestrial and oceanic- is strongly influenced by the weather conditions and by the considerable
supplies of mineral particles, including events with high intensity due to an unusual volume of
airborne dust from Sahara.
The second conclusion is about this research field which study, since several years ago, the
complex relationships between different climatic factors.
Thirdly, the geological importance of Sahara dust deposition is evident after studying the
mineral composition of soils and also several particular types of landforms. Evidences for a
historical time and today is a future research line.
Fourth, this kind of study, using long statistical series of meteorological data and mineral
composition of dust, reveals the existence of tendencies or cycles in this kind of atmospheric
behaviour which together with data from other measurements, understanding the problem of
Climatic Change. In this way, some climatic models, aids in Hadley Centre (United Kingdom),
point to a future intense heating of Sahara desert. This, which can be expected to produce more
powerful expulsions of continental air and an even larger supply of dust to the atmosphere than
today, with the resultant increase of forest fire risk.
Finally, our research provides data for future research on the effects of Sahara dust on the
human health, and on the tourism, the main economic activity of the Canary.
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