Granulometric analysis of aeolian dust deposits with Malvern Morphologi G3-ID

 

Automated imaging provides a unique technique to gather direct information on granulometric characteristics of sedimentary particles. Granulometric data obtained from automatic image analysis of Malvern Morphologi G3-ID is a previously does not applied, brand new technique for particle size and shape analyses in sedimentary geology. Size and shape data of several hundred-thousand individual particles were automatically recorded for each sample from the captured high-resolution images. Several size (e.g. circle-equivalent diameter, major axis, length, width, area) and shape parameters (e.g. elongation, circularity, convexity) were calculated by the instrument software. While the mean light intensity after transmission through each particle is automatically collected by the system as a proxy of optical properties of the material.

Effects of particle optical properties on grain size measurements of aeolian dust deposits

Particle size data are holding crucial information on the sedimentary environment at the time the aeolian dust deposits were accumulated. However, only the correct granulometric data and its proper interpretation can be used in this kind of investigations.

Various aspects of aeolian sedimentation (wind strength, distance to source(s), possible secondary source regions and modes of sedimentation and transport) can be reconstructed from proper grain size distribution data. Laser diffraction methods provide much more accurate and reliable information on the major granulometric properties of wind-blown sediments compared to the sieve and pipette methods. 

The Fraunhofer and the Mie optical theories have been used for laser diffraction grain size measurements. The two different approaches need different “background” information on the measured medium. During a measurement with Fraunhofer theory, it is assumed that the particles are relatively large (over 25-30 µm – about 40 times larger than the wavelength of the ligth) and opaque. The Mie theory could offer more accurate data on the smaller fraction (clay and fine silt), but the knowledge of the refraction and absorption indices is needed (as it is a solution for Maxwell's electromagnetic field equations), which is a difficult question in the case of polymineralic samples.

Springtime conferences in Vienna

Dan H. Yaalon Symposium

Vienna, Austria

Date: April, 8th - 11th, 2015

Venue: Boku-Vienna/ Universität für Bodenkultur

Multidisciplinary investigations of Saharan dust events in the Mediterranean and in the Carpathian Basin (Central Europe)

Several hundred tons of windblown dust material is transported every year from Saharan dust source areas into direction of Europe, modifying important climatic and other environmental processes of distant areas. Past Saharan dust addition has also played crucial role in the unique Mediterranean terra rossa formation.

NASA’s daily aerosol indices (from 1979 to 2012) were employed to estimate atmospheric dust amount in the investigation areas. Daily geopotential height (at 700 mb), wind vector and meridional flow data of the distinguished dust events were obtained from the NCEP/NCAR Reanalysis project to compile mean synoptic composite maps. In order to identify the transportation routes and source areas, the backward trajectories were plotted on multiple trajectory maps (NOAA HYSPLIT model). 

The main period of the dust transportation into the Mediterranean is in spring and summer, when the thermal convective activity forces the injection of particles to higher atmospheric levels. However, seasonality patterns of the different Mediterranean sub-basins show quite large differences. In western sub-basins, the maxima of Saharan dust outbreaks is in summer, in the eastern basin, dust storms occur typically in spring, while the seasonal distribution of dust events in the central sub-basins show typical bimodal characteristic with a spring and summer peak, indicating a transitional position.

In the Carpathian Basin, the generally spring and summer dust events can be connected to three different synoptic situations: (1) SW flow between a southward moving trough (over the Atlantic coast of Africa) and the eastern cell of the divided subtropical high; (2) southerly winds associated with eastward moving depressions; and (3) long-range transport from the western parts of the Sahara along the western fringe of an anticyclone and by the westerlies. Granulometric properties of recently deposited dust material were analysed by using electron microscope (SEM) and Malvern Mastersizer 3000 (Hydro LV) laser particle size analyser.

Dust activity of Saharan sources has been much more dominant during certain Pleistocene glacial periods, as it is inferred by the widespread aeolian dust deposits of the investigation area with relevant Saharan contribution. The Saharan dust flux has decreased during the interglacials compared to glacials. However, according to Eastern Mediterranean marine drillings, at the end of the Early Pleistocene Saharan dust deposition was also significant during interglacial periods. It was the time of the formation of red paleosoils in the Carpathian Basin. The fine-grained dust originating from the Sahara could be incorporated to the MIS-17 and MIS-19 soils. 

Grain size, geochemical, mineralogical, micromorphological and SEM analyses have been elaborated on major Hungarian red clay-loess-palaeosol sequences to get more information on aeolian dust deposition and its role in syngenetic, accretionary soil development even during periods characterized by relatively higher temperatures and rainfall. The detailed granulometric analyses of the red clays and red paleosoils represent similarity in terms of their bimodal grain-size distribution patterns with loess horizons. The geochemical analyses suggest that climate during the formation of red clays and red paleosoils was considerably more humid and warmer in comparison to younger interglacials or to modern values.

Support of the Hungarian Research Fund OTKA under contract PD108708 (for G. Varga) is gratefully acknowledged. It was additionally supported (for G. Varga) by the Bolyai János Research Scholarship of the Hungarian Academy of Sciences.

European Geosciences Union General Assembly 2015

Vienna, Austria

Date: April, 12th - 17th, 2015

Venue: Austria Center Vienna

The EGU General Assembly 2015 was again a great success with 4,870 oral, 8,489 poster, and 705 PICO presentations. 577 unique scientific sessions together with 310 side events created an interesting programme. At the conference 11,837 scientists from 108 countries participated, of which 23% were students, 15,000 copies of EGU Today distributed, keen media presence and reporting, and thousands of visits to the webstreams as well as to the EGU blog GeoLog. 

Effects of particle optical properties on grain size measurements of aeolian dust deposits

Particle size data are holding crucial information on the sedimentary environment at the time the aeolian dust deposits were accumulated. Various aspects of aeolian sedimentation (wind strength, distance to source(s), possible secondary source regions and modes of sedimentation and transport) can be reconstructed from proper grain size distribution data. Laser diffraction methods provide much more accurate and reliable information on the major granulometric properties of wind-blown sediments compared to the sieve and pipette methods. 

The Fraunhofer and Mie scattering theories are generally used for laser diffraction grain size measurements. The two different approaches need different ‘background’ information on the medium measured. During measurements following the Fraunhofer theory, the basic assumption is that parcticles are relatively large (over 25-30 µm) and opaque. The Mie theory could offer more accurate data on smaller fractions (clay and fine silt), assuming that a proper, a’priori knowledge on refraction and absorption indices exists, which is rarely the case for polymineral samples. This study is aimed at determining the effects of different optical parameters on grain size distributions (e.g. clay-content, median, mode).

Multiple samples collected from Hungarian red clay and loess-paleosol records have been analysed using a Malvern Mastersizer 3000 laser diffraction particle sizer (with a Hydro LV unit). Additional grain size measurements have been made on a Fritsch Analysette 22 Microtec and a Horiba Partica La-950 v2 instrument to investigate possible effects of the used laser sources with different wavelengths. XRF and XRD measurements have also been undertaken to gain insight into the geochemical/mineralogical compositions of the samples studied.

Major findings include that measurements using the Mie theory provide more accurate data on the grain size distribution of aeolian dust deposits. Significant differences between the Mie and Fraunhofer approaches have been found for the finest grain size fractions, while only slight discrepancies were observed for the medium to coarse silt fractions. Since the two approaches gave similar results for the medium to coarse silt fractions that are the most abundant in loess, the use of the different approaches has no appreciable effect on the mode of the distributions. In conclusion, the two different applied optical parameter settings has had significant effects on the finer tail of the aeolian grain size distributions.

Support of the Hungarian Research Fund OTKA under contracts PD108708 (for G. Varga) and PD108639 (for G. Újvári) are gratefully acknowledged. It was additionally supported (for G. Varga) by the Bolyai János Research Scholarship of the Hungarian Academy of Sciences.

Various environments of interglacials recorded by Pleistocene paleosoils in Hungary (Central Europe)

Based on stable isotope analyses of worldwide reference curves from deep sea, ice core and speleothem records, it has long been apparent that duration, intensity and climatic conditions of different interglacial periods were significantly diverse. As a consequence of negligible fresh, detrital material admixture during interglacials, the soil formation intensity and maturity of various kinds of past soils have been holding vital information on the environmental conditions at the time the soils formed. This, in turn, means that several physical and chemical properties of soils allow us to reconstruct past climatic regimes.

Loess-paleosol sequences in Hungary (Central Europe) provide insight into the cyclic nature of glacial-interglacial variations of the last 1 million years. The paleosoils have been recognized as the product of warmer and moister interglacials, when the (glacial) loess material was altered by chemical weathering and pedogenic processes. 12 pedogene units from MIS-19 to MIS-5 strata were analysed in the course of this study, with a special attention to MIS-11 and MIS-19 periods, because of these can be regarded as analogues of the Holocene interglacial (due to the similarities in obliquity and eccentricity). Grain size, geochemical and (clay)mineralogical studies were elaborated and were gathered from previously published papers to quantify past weathering intensity and paleoenvironmental conditions by geochemical climofunctions.

The Upper and partly, the Middle Pleistocene loess deposits are intercalated by steppe, forest-steppe and brown forest soils, while the older pedogene horizons are different kinds; these are red, Mediterranean-type soils. The MIS-5 pedocomplex consist of three parts at several Hungarian sites, however the pedogene units cannot be correlated unequivocally with the three MIS-5 warmer substages, due to the scarce absolute age data. The MIS-7 and MIS-9 stages are represented by three forest steppe soils. The MIS-11 pedocomplex and MIS-17 and MIS-19 units are thick and well-developed forest soils, formed under a more humid climate compared to the younger pedogene strata. The geochemical data and climofunctions have been supported well these findings. Contrary to the global loess-paleosoil sequences, the MIS-13 and MIS-15 soils are not so dominant in the Hungarian series.

Support of the Hungarian Research Fund OTKA under contract PD108708 (for G. Varga) is gratefully acknowledged. It was additionally supported (for G. Varga) by the Bolyai János Research Scholarship of the Hungarian Academy of Sciences.

DUST2014 - International Conference on Atmospheric Dust

The meeting was organized by the Associazione Italiana per lo Studio delle Argille - onlus (AISA, Italian Association for the Study of Clays) and the Istituto di Metodologie per l’Analisi Ambientale (IMAA, Institute of Methodologies for Environmental Analysis), National Research Council of Italy (CNR). The scientific program of the meeting included 5 plenary lectures, 313 oral and 153 poster contributions. All the contributions were revised by 72 international experts, many of them being the conveners of the 34 sessions dealing with the seven themes of the Conference: Chemical & Mineralogical Studies, Geological Records, Health & Environment, Instrumentations & Measurements, MF: Modelling & Field Studies, The Universe of Dust - General Session.

ORAL PRESENTATION: Aeolian dust derived Plio-Pleistocene re palaeosols in the Carpathian Basin

Aeolian dust deposits, covering more than half of the Carpathian Basin in Central Europe, form thick (up to 50 m) loess-paleosol sequences that are generally underlain by terrestrial red clays. These deposits provide information on the palaeoclimatic and palaeoenvironmental changes of the last 3–3.5 million years. The Upper and partly, the Middle Pleistocene loess deposits are intercalated by steppe, forest-steppe and brown forest soils, while the older pedogene horizons are different kinds; these are red, Mediterranean-type soils. The younger soils were formed from the underlying loess deposits, while according to preliminary data, interglacial dust deposition could have played more dominant role during the formation of the older ones, similarly to certain types of red clays. Grain size, geochemical, (clay) mineralogical, micromorphological and scanning electron microscopic (SEM) analyses have been elaborated on major Hungarian red clay-loess-palaeosol sequences to get more information on aeolian dust deposition and its role in syngenetic, accretionary soil development even during periods characterized by relatively higher temperatures and rainfall.

The detailed granulometric analyses of the red clays and red palaeosols represent similarity in terms of their bimodal grain-size distribution patterns with loess horizons, while the SEM images also show an aeolian origin of the extracted quartz grains characterized by sharp edges, breaks and stepped surfaces. The geochemical and (clay) mineralogical analyses suggest that climate during the formation of red clays and red palaeosols was considerably more humid and warmer in comparison to younger interglacials or to modern values.

NEW Paper - Saharan dust in the Mediterranean region

Varga, Gy., Újvári, G., Kovács, J. (2014). Spatiotemporal patterns of Saharan dust outbreaks in the Mediterranean Basin. Aeolian Research 15. pp. 151–160.

Saharan dust outbreaks transport appreciable amounts of mineral particles into the atmosphere of the Mediterranean Basin. Atmospheric particulates have significant impacts on numerous atmospheric, climatic and biogeochemical processes. The recognition of background drivers, spatial and temporal variations of the amount of Saharan dust particles in the Mediterranean can lead to a better understanding of possible past and future environmental effects of atmospheric dust in the region.

For this study the daily NASA Total Ozone Mapping Spectrometer's and Ozone Monitoring Instrument’s aerosol data (1979– 2012) were employed to estimate atmospheric dust amount. Daily geopotential height, wind vector and meridional flow data of the distinguished dust events were obtained from the NCEP/NCAR Reanalysis to compile mean synoptic composite maps. In order to identify the typical dust transportation routes and possible source areas, the backward trajectories were plotted using the NOAA HYSPLIT model.

The main period of the dust transportation is from March to end of August, when the thermal convective activity forces the injection of particles to higher atmospheric levels. However, seasonality patterns of the different Mediterranean sub-basins show quite large differences. In western sub-basins, the maxima of Saharan dust outbreaks is in summer, related southwest flow between a southward emanating trough and the northward migrating subtropical high-pressure centre. In the eastern basin, dust storms occur typically in spring, generated by the warm sector winds on foreside of eastward moving Mediterranean and Sharav cyclones. The seasonal distribution of dust events in the central sub-basins shows a bimodal characteristic with a spring and summer peak.

Saharan dust addition plays crucial role in the unique Mediterranean terra rossa formation too, where the chemical compounds of soils (e.g. silt sized quartz in limestone or basalt derived soils) can only be explained by some external, aeolian dust accretion as it was identified in Portugal (Jahn et al., 1991), in Spain (Muhs et al., 2010), in Italy (Jackson et al., 1982), in Croatia (Durn et al., 1999), in Greece (MacLeod, 1980), in Turkey (Atalay, 1997) and in Israel (Yaalon and Ganor, 1973; Yaalon, 1997). Small pulses and near-continuous dust addition to soil could affect the whole texture, individual horizons and the fertility by dust-derived nutrients and clay minerals (Simonson, 1995).

Dust activity of Saharan sources has been much more dominant during Pleistocene glacial periods, as it is inferred by the widespread aeolian dust deposits (loess, desert loess, loess-like deposits and marine sediments) of the investigation area with relevant Saharan contribution (Tsoar and Pye, 1987; Cremaschi, 1990; Rózycki, 1991; Hoogakker et al., 2004; Larrasoaña et al., 2008).

NEW paper on Last Interglacial dust forming mechanisms (e.g. syngenetic dust addition)

Bradák, B., Kiss, K., Barta, G., Varga, Gy., Szeberényi, J., Józsa, S., Novothny, Á., Kovács, J., Markó, A., Mészáros, E., Szalai, Z. (2014). Different paleoenvironments of Late Pleistocene age identified in Verőce outcrop, Hungary: Preliminary results. Quaternary International 319. pp. 119–136.

Complex geomorphological, geological, palaeopedological and chronometrical investigations were started to reveal the development of the alluvial section and the loess/paleosol sequence containing remnants of a Late Palaeolithic site near Verőce, Hungary.

Different palaeoenvironments were identified in the profiles of the abandoned brickyard influenced by different facies in the margin of fluvial-alluvial (Palaeo-Danube) and proluvial (South East pediment of Börzsöny Mountain) area and the environments affected by the climate fluctuation of Late Pleistocene (loess/paleosol sequence overlaying the base of alluvial materials). 

Sediments possibly deposited by Palaeo-Danube were identified in a basal section of abandoned brickyard.  The alluvial facies was indicated by sand, aleurite and clay layers in the sediment sequence. Some parts of the alluvial sediments were covered by loess in the glacial periods intercalated by four paleosol horizons formed during interglacial or interstadial periods. The sedimentation of loess and the forming of paleosols were finished by pedimentation processes (sheet wash, redeposition) indicated by the fine layered material at the top of each paleosol horizon.  

Another complex fluvial-alluvial section was identified at a different part of the brickyard possibly developed parallel with the loess/paleosol sequence in the Late Pleistocene.

Based on the different kind of dating methods (e.g. archaeological, 14C and luminescence dating) the development of the loess/paleosol sequence started in marine isotope stage 6 (MIS6) and the youngest layer dated back to MIS2.

Samples were taken from all main stratigraphic units of the outcrop for grain size analysis. The grain-size distribution of collected loess, paleosol and loess-like samples were measured in the laboratory of the Geographical Institute of the University of Pécs by using a Fritsch Analysette 22 Compact laser grain-size analyser, with 0.3–300 μm measurement ranges in 62 channels. Grain-size measurements by laser particle sizers provide more accurate results than the previously applied pipette and sieve methods (Konert and Vandenberghe, 1997; Blott and Pye, 2006).

The chemical extraction procedure described by Konert and Vandenberghe (1997) was applied before the measurements to sequentially remove the organic material and carbonate content by treating the samples (3 g) with H₂O₂ (10 ml, 30%) and HCl (10 ml, 10%). After desiccation of samples, sodium hexametaphosphate ((NaPO₃)₆) was added in order to disperse the particles.

Mineral particles of the samples are generally falling into clay, fine and coarse silt ranges, occasionally with minor fine sand components (Fig. 3). Most of the measured loess grain-size distribution curves (from L1–5) have very similar shape patterns with definite positive skewness (asymmetry into the direction of coarse fractions), unimodality (or weakly developed bimodality) and leptokurtic kurtosis. Paleosols (P1–4) have more diverse granulometric profile; for instance, curves of red pedogene horizons of P4 units have almost the same characteristics as the underlying loess deposits with a slightly more finer components, while grain-size distributions of brownish and especially blackish levels shift into the direction of clay and very fine silt fractions occasionally without any coarse silt particles, mesokurtic kurtosis and negative skewness. Redeposited loess-like (L1r–L4r) and aleurite samples (A1) show more various properties with bi- or trimodal distributions, moderate kurtosis and normal or negative skewness. Samples from section B contain lot of coarse silt- and sand-sized aggregates of sedimentary clay and fine silt particles. Presumably, these aggregates are connected to post-depositional processes and/or have been formed by redeposition. According to Mason et al. (2003, 2011) and Qiang et al. (2010), the genetic explanation of aggregates is highly uncertain.

Deposits, described as loess (e.g. B1-11; C1-15; D2-10,11) during the fieldworks were confirmed by the grain-size analysis. The granulometric profile with a pronounced maximum in medium and coarse silt fraction and a tail or shoulder in the clay and fine silt components indicate a primary aeolian depositional environment (Nugteren et al., 2004; Vriend and Prins, M.A. 2005, Vandenberghe et al., 2006; Sun et al., 2008; Novothny et al., 2011; Varga et al., 2012). General characteristics of grain-size distribution curves represent a similar sedimentary origin of the analysed samples; however they differ largely from fluvial, lacustrine or other hydraulic deposits.

The loess-like and aleurite deposits can be regarded as typical redeposited sediments due to their layered deposition, multimodal grain-size distribution and poor sortness.

The L5 loess and the overlying P4 paleosol levels of the different sections could be well correlated. The granulometric profile (lower average and modal grain-size, higher clay-content, slightly modest skewness and kurtosis compared to underlying loess) and stratigraphic position of red paleosol horizons of P4 paleosols are indicating an in-situ origin of the paleosol. These strata were formed from the L5 loess deposits. (Similar stratigraphic connection among loess and other, younger pedogene horizons (e.g. P2, P3) could not be identified in the sections.) The investigation of the pedogenesis of upper levels of P4 is much more complicated based on the grain-size analysis; probably mass movements, redeposition and sheet wash processes played more significant role in formation of these brownish and reddish horizons. Especially, in the case of black soil-layers (B1-7; C1-13; D2-4), the fairly high clay-content and the almost totally missing coarser fractions are indicating well the redeposited origin. After the pedogenic phase dust accumulation became dominant again due to the changing climatic conditions. During dust settlement the microecosystem of the soil remained still active and tried to keep pace with the accumulation rates. This ensured the presence of pedogenic processes in the dust accumulation phase as well (Catt 1990, Becze-Deák et al. 1997). The SDI shows similar value to the P4 paleosol identified in the B profile.

Aeolian dust deposits in Hungary - a general description

Wind-blown loess and loess-like deposits are widely distributed in the Carpathian Basin, covering more than half of the area. Traditionally, based on its lithology, five main units have been distinguished; the Dunaújváros–Tápiósüly series and the Mende–Basaharc series belong to the young loess sediments, the Paks I. and Paks II. series belong to the old loess sediments, while the oldest strata of the sequence is part of the Dunaföldvár series. This last section consists of thin loess horizons between red (Mediterranean-type) palaeosols, reddish clays and loess-like deposits, underlain by red clay.

The upper three series can be correlated well with marine isotope stages (Gábris, 2007), and also with the Malan, Upper Lishi and partially with the Lower Lishi series in the Chinese Loess Plateau. The correlation with regional sequences, e.g. Serbian loess deposits (Markovic, S.B. et al., 2006, 2009), according to the stratigraphic similarities is also possible. These sediments can be regarded as the typical, glacial–interglacial loess–palaeosol successions.

The older loess and loessial section in the Carpathian Basin is known only from some exposures and boreholes. The lithological properties (reddish-yellow colour, massive structure, smaller thickness of loess layers between red palaeosols, large carbonate concretions) of these strata are very similar to the oldest sequences in Central Asia and to the Wucheng loess in East Asia.

The loess–palaeosol sections are generally underlain by red clay, which aeolian origin in Hungary according to the new data (Kovács, 2008; Kovács et al., 2008) is suggested. Thus, red clay can be considered as mature, accretionary palaeosol-complex/pedocomplex.

Red clay deposits under loess (Photo: Kovács, J.)

The red clay–loess–palaeosol record in the Chinese Loess Plateau indicates that the dust deposition started ~8 Ma in the area. Dust record of the ODP 885/885 site (North Pacific Ocean) show strong similarity in terms of their mean linear sedimentation rate pattern, indicating a period of increasing dust production in the inner part of the Asian continent (Rea and Hovan, 1995; Rea et al., 1998). Whereas, during the warm–moist Pliocene climate, the extent of the source area and the dust accumulation-rate were smaller, while the postdepositional pedogenic processes were stronger as compared to the Pleistocene conditions. The shift from this moist and warm climate to a more arid one allowed of the formation of the oldest loess deposits around 2.6 Ma.

On the role of desert dust and dust storms in environmental processes

Dust storms and related atmospheric mineral particles have been in the focus of environmental and climatic studies for the last two decades (Stout et al., 2009). Previous investigations confirmed that windblown dust is an active component of the climate system, and can modify its elements via both direct and indirect effects (Harrison et al., 2001, Kohfeld and Tegen, 2007, Maher et al., 2010 and Pósfai and Buseck, 2010). Dust particles affect the Earth's energy balance directly through absorption, scattering and reflection of incoming shortwave and outgoing longwave radiation or by changing the albedo of (bright) surfaces (e.g. Arimoto, 2001). Indirectly, by acting as cloud condensation nuclei, mineral particles have also an effect on atmospheric moisture balance (Rosenfeld et al., 2001 and Sassen et al., 2003). Particles rich in Fe have major impact on iron-limited oceanic ecosystems, and thus, dust can influence the primary phytoplankton production and the carbon cycle through biogeochemical interactions (Ridgwell, 2002).

The global annual input of mineral dust deflated from arid-semiarid areas can be set in the range between 1 and 3 billion of tons (Tegen et al., 1996, Mahowald et al., 1999, Mahowald et al., 2006, Ginoux et al., 2001 and Mahowald et al., 2006). Most important sources are situated in Saharan and Sahel regions, which are responsible for 50–70% of the global emission (Ginoux et al., 2001 and Miller et al., 2004). Four main pathways of Saharan dust transport can be distinguished: (1) southward to Gulf of Guinea; (2) westward over the North Atlantic Ocean; (3) eastward to Middle East; and (4) northward to Europe (for more details, see Engelstaedter et al., 2006 and Goudie and Middleton, 2006).

The several hundred thousand tons of dust derived from Saharan sources influence numerous constituents of European environmental systems (D'Almeida, 1986 and Prospero, 1996). During heavy dust-outbreaks, atmospheric dust concentration often exceed PM10 standards of the European Union in Spain (Rodríguez et al., 2001), in Italy (Matassoni et al., 2011) and in Greece (Gerasopoulos et al., 2006), thereby affecting human health (Griffin et al., 2001). The strongly alkaline dust particles increase the pH of precipitation, thus reduce the frequency of acid rains (Roda et al., 1993, Rogora et al., 2004 and Špoler Čanić et al., 2009). As proposed by Psenner (1999), permanent Saharan dust contributions to low-alkalinity European lakes prevented them to become acidic during the late twentieth century. The accumulated dust particles are even capable of modifying soil properties of a given region (Yaalon, 1997). As such, terra rossa soils in Portugal (Jahn et al., 1991), in Spain (Muhs et al., 2010), in Italy (Jackson et al., 1982), in Croatia (Durn et al., 1999), in Greece (MacLeod, 1980) and in Turkey (Atalay, 1997) have been shown to be an alteration product of local material and far-travelled African mineral dust.

Fine-grained particles lift to higher levels of the atmosphere and have a long atmospheric residence time up to a few weeks (Pye, 1987). Aeolian dust from North Africa can often be detected in Europe's high-latitude areas e.g. in British Isles (Wheeler, 1986), in Germany (Klein et al., 2010), in Scandinavia (Franzén et al., 1994 and Barkan and Alpert, 2010) and even in our study area, the Carpathian Basin (CB) (Central Europe) (Borbély-Kiss et al., 2004, Koltay et al., 2006 and Szoboszlai et al., 2009). Nowadays the CB is generally not regarded as a dusty place, except for episodic dust storms related to cold fronts invading the region at the beginning of the vegetation period in the early spring. Still, Central Europe is lying in the D1b zone of the “Saharan dust-fall map” of Stuut et al. (2009), implying that recent Saharan dust material can be incorporated into the soil system and may increase its fine silt content (Stuut et al., 2009). However, dust activity of the region was much more significant during the Plio–Pleistocene periods, as it is shown by thick aeolian dust deposits covering more than half of the area (e.g. Pécsi and Schweitzer, 1993, Kovács et al., 2008, Újvári et al., 2010, Kovács et al., 2011 and Varga, 2011). It has been recognized that mineral dust particles of these aeolian sediments originate mainly from local sources (e.g. alluvial plains), and only the clay and fine-silt fractions may be linked to Saharan sources (Rózycki, 1991, Rousseau et al., 2007 and Újvári et al., 2012), similarly to Italian loess deposits (Cremaschi, 1990a and Cremaschi, 1990b).

Terra rossa sampling in Croatia (Istria)

Terra rossa (red Mediterranean soil) is a reddish clayey to silty-clay material, which covers limestone and dolomite in the form of a discontinuous layer ranging in thickness from a few centimetres to several metres (Durn, 2003).

The origin of the material of Terra rossa is a long-standing problem. According to several researchers, the parent material of this unique soil formation could have been originated from deposited aeolian dust, which in the Mediterranean region originates from the Sahara.

Terra rossa covers large areas also in the northern part of the Adriatic Sea, at the Istrian Peninsula, from where red soil samples have been taken in the course of the OTKA project, in October 2013:

NEW Research Project: The role of dust storms and aeolian dust deposition in the formation of Central European red clays and red paleosoils in loess sequences

New research project entitled "The role of dust storms and aeolian dust deposition in the formation of Central European red clays and red paleosoils in loess sequences" has started.

Project start: 01/09/2013

Closing date: 31/08/2016

Summary of the research and its aims for experts

The aim of the proposed research is the investigation of role of dust storms and aeolian dust sedimentation in the formation of red clays and red paleosoils of loess sequences. Almost half of the Carpathian Basin is covered by thick loess-paleosoil series, generally underlain by red clays. These deposits provide information on the paleoclimatic and paleoenvironmental changes of the last 3–3.5 million years. The Late and partly, the Middle Pleistocene loess deposits are intercalated by steppe, forest-steppe and brown forest soils, while the older pedogene horizons are different kinds; these are red, Mediterranean-type soils. The younger soils were formed from the underlying loess deposits, while according to preliminary data, dust deposition could have played more dominant role during the formation of the older ones; similarly to certain types of red clays.

In the course of the proposed research, red clay-loess-paleosoil samples from the Carpathian Basin and Saharan dust-derived “terra rossae” from the Mediterranean (e.g. Istria, Croatia) would be investigated by grain-size (granulometric profile for sedimentation type), magnetic susceptibility (environment, startigraphy), scanning electron microscopy (micromorphology, transportation), geochemical and clay-mineralogical (provenance, environment) analyses to get more information on the origin of the deposits, with respect to paleocirculation data. By using these various types of data and stratigraphic survey, I would like to refine the results of previous paleoenvironmental reconstructions (e.g. climofunctions, aeolian dust concentration), which have not taken into account the possibility of interglacial dust deposition.

What is the major research question?

In the course of the investigations, I would like to study the intensity of interglacial aeolian dust deposition, which could have been sufficient to syngentic, accretionary soil development even during the warm-moist periods. The similarities of Saharan dust-derived Mediterranean terra rossae and red paleosoils in the Carpathian Basin are also in the focus of this project.

I would like to determine the ratio of the two, distinct types of aeolian dust sedimentation mechanisms (discontinuous dust storms and almost persistent back-ground dust loads), which were identified during observations of recent dust depositional episodes in several samples. By mathematical-statistical partition of polymodal grain-size distribution curves and by the concrete separation of the sediment populations, the coarse particles from local sources transported by short suspension episodes and the fine-grained ones from large distances could be analysed separately, providing more information on the provenance. The question of detrital and secondary origin of clay and fine-silt sized particles can be answered by the comparison of sedimentary data of fine-grained populations from distinct sources and ages, and by the observations of recent dust intrusions.

What is the significance of the research?

The possibility of significant interglacial aeolian dust deposition is leading to several other questions. According to the classical assumption, the loess deposits have been formed from the depositing dust material, while the paleosoils developed from the underlying loess deposits by weak weathering processes. However, intensive interglacial dust accumulation claims a different kind of stratigraphic interpretation. In the first case, when the soils were formed from the underlying deposits, the last period of loess formation could not have been identified as loess in the sequence. In the second case, the soils form syngenetically from the falling dust, and all of the changes are represented in the stratigraphic column. From a paleocliamtic viewpoint, these glacial-interglacial shifts and abrupt warmings of glacial climax periods are one of the most interesting research topics.

The paleoprecipitation and paleotemperature data of the widely used geochemical climofunctions deserve also further reconsideration. The fine-grained populations of deposits are consisting of detrital and secondary particles; only the secondary ones provide relevant information on the environmental properties of the soil formation. By the assessment of the amount of detrital, windblown clay-minerals the result of these reconstructions could have be refined significantly.

Dust storms and related atmospheric mineral dust particles have been standing in the focus of environmental studies for the last two decades. Investigations confirmed that aeolian dust is an active component of the climate system, and can modify its elements via both direct (e.g. modifying the incoming solar irradiance and outgoing longwave radiation) and indirect (e.g. iron fertilization, cloud-formation); however our knowledge on these topics is still insufficient. In some periods of the Earth’s history the amount of atmospheric aeolian dust have increased by several orders of magnitude, compared to the present 1–2 billion of tons, and played substantial role in the climatic system.

By the comparison of reconstructed interglacial long-range dust transportation and recent observations, the anthropogenic responsibility for dust outbreaks can be evaluated.

Summary and aims of the research for the public

Almost half of the area of the Carpathian Basin is covered by thick loess deposits. The pale yellow material has been accumulated by intensive Pleistocene dust storms. The cold-dry climate was not suitable for pedogenic processes, however during the moist and warm interglacial periods the upper part of the loess has been formed to soil. These dark layers were buried by dust during the next glacial phase, and archived the warm interglacial period as paleosoil. (The most famous Hungarian loess-paleosoil sequences are situated along the River Danube; e.g. the high-bluffs of Dunaföldvár, Paks, Dunaszekcső.)

The warming periods were not uniform, during different periods different types of soils have been formed. The younger loess deposits are intercalated by steppe-like soils, while the older (more than 650 kyr old), red paleosoils have been formed under a Mediterranean-like climatic regime. These old paleosoils have some similar properties to the red clay deposits, which are underlying the lowermost, oldest loess deposits and have been formed under a warmer and moister climate.

According to the preliminary data, aeolian dust played important role in the sedimentation during the formation of red clays and red paleosoils. By using different sedimentary and stratigraphic methods, the transportation-type and origin of soil’s material, and also the environment of their formation can be determined.

Geographical distribution and geomorphological characteristics of major global dust source areas

Satellite measurements provide useful information on mineral aerosol content of the atmosphere and on distribution of dust source regions. The NASA Total Ozone Mapping Spectrometer’s (TOMS) and Ozone Monitoring Instrument’s (OMI) Aerosol Index measurements (on board of different sun-synchronous satellites) have the longest (since November 1978) available global record with appropriate spatial (1×1.25 and 1×1 degree) and temporal (daily) resolution. The data-matrix was analysed by a self-developed mathematical-statistical MATLAB algorithm to attain information on situation and inter-annual and seasonal activity of major dust hot-spot areas.

The compiled average aerosol maps show that the most important sources are situated mainly in the desert, semi-desert regions of North Africa, Middle East and Central Asia creating a more or less continuous area, called “Global Dust Belt”. The frequency and magnitude of dust emission outside the dust belt is relatively low, and is concentrated in small distinct areas. In general, the temporal activity of source regions has an obvious seasonal pattern with a spring-summer maximum.

Based on the systematic analyses of geographical distribution, it can be stated that major sources are associated to specific geomorphological environments. These can be connected to geomorpholocial depressions, ephemeral streams or wadi-systems and to alluvial fans. The fine-grained material of most of the sources was accumulated in some kind of fluvial or lacustrine environment with a certain Pleistocene pluvial history, which acts as a dust source area after the desiccation.

Figure 1. Global mean map of the measured daily Aerosol Index values and locations of the discussed major dust source domains (modified after VARGA, GY. 2012). The investigated dust hot-spots are the followings: 1. North Africa: 1.1. Bodélé Depression; 1.2. Azawagh Structural Basin; 1.3. Taoudenni Basin; 1.4. W-Saharan sebkhas; 1.5. Tidikelt Depression; alluvial fans and wadis of W/NW-Ahaggar; 1.6. Chott Melrhir and Chott Jerid; 1.7. Cyreneica és a Quattara Depression; 1.8. Western escarpments of River Nile; 1.9. Tokar Delta; 2. SW Asia: 2.1. Salt flats and ephemeral streams of Jebel Tuwaiq; 2.2. Sabkha system of Jebel Dhoraf; 2.3. Floodplains deposits and marshlands of Tigris and Euphrates Basin; 3. Central Asia: 3.1. Sistan (Seistan) Basin; 3.2. Dasht-e Kavir and Dasth-e Lut; 3.3. Fergana Basin; 3.4. Kara-Bogaz Gol; 3.5. Aral Sea; 3.6. Alluvial deposits of Balkhas-Alakol Basin; 3.7. Junggar Basin; 3.8. Uvs Lake Basin; 3.9. Tarim Basin; 3.10. Salt flat of Lop Nor and Quaidam Basin; 3.12. Thar; 4. North America: 4.1. Salt flats at Great Salt Lake; 4.2. Smoke Creek and Black Rock Deserts; 4.3. Playa system of Salton Sink; 4.4. Chihuahuan Desert; 4.5. Bolsón de Mapimí; 5. South America: 5.1. Salar de Uyuni (and other salt lakes/salars of the Altiplano); 5.2. Eastern slopes of Southern Andes; 5.3. Patagonia; 6. South Africa: 6.1. Etosha Pan; 6.2. Makgadikgadi Depression; 7. Australia: 7.1. Lake Eyre Basin; 7.2. Floodplain deposits and salt swamps along River Darling (e.g. Caryapundy); 7.3. Floodplain deposits and salt swamps along River Murray; 7.4. Barkly Tableland. 

Full paper: Varga, Gy., Bradák, B., Szeberényi, J. (2014). Geographical distribution and geomorphological characteristics of major global dust source areas. In: Jakab, G., Szalai, Z. (Eds.) Talajpusztulás térben és időben: az "Eróziós kerekasztal 2013" közleményei. pp. 40–46.