Saharan dust intrusions toward the Carpathian Basin
Aeolian dust particles and dust storms play substantial role in climatic and other environmental processes of the Earth system. The largest and most important dust source areas are situated in the Sahara, from where several hundred thousand tons of mineral dust is emitted each year and transported towards the European continent. Here we show that 130 Saharan dust events (SDEs) reached the atmosphere of the Carpathian Basin from 1979 to 2011 by using the NASA’s daily TOMS Aerosol Index data, satellite images and backward trajectory calculations of NOAA HYSPLIT model. Monthly trends of dust events demonstrate that the main period of dust transportation is in the spring, with a secondary maximum in the summer (in July and August). This seasonal distribution match well the seasonality of Saharan dust emissions. However synoptic meteorological conditions govern primarily the occurrence of long-range dust transport towards Central Europe. Based on their different meteorological backgrounds (geopotential field, wind vector and meridional flow), SDEs were classified into three main types. By using composite mean maps of synoptic situations and backward trajectories, the possible source areas have also been identified for the different types of events. Finally, we provide a short discussion on how the African mineral dust could contribute to the local aeolian sedimentation of the Carpathian Basin during the Plio-Pleistocene.
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.
The widely distributed Plio–Pleistocene aeolian dust deposits are one of the most important terrestrial archives of past climate and environmental changes. The alternation of loess and palaeosol layers is regarded as evidence of the cyclic nature of Pleistocene climate changes. The loess succession is underlain by aeolian red clay, which has been formed under warm-humid climate.
According to the studies of red clay–loess–palaeosol sequences from China, Central Asia, Alaska, South America and Central Europe, the mineral dust deposition has shown similar pattern since the Pliocene. The typical loess of the last one million years reflects glacial–interglacial conditions, whereas the old loess–palaeosol sequences could be the product of shorter arid–humid climate cycles. The similarity of the sedimentary structures could be caused by the similar global climate conditions, controlled by orbital forcing.
The investigated sections from Hungary (Central Europe) have been affected by local and regional geomorphological and climate factors. Even so, they can be correlated fairly well with the major global climate changes. The Hungarian aeolian dust deposits consist of three main groups of sedimentary formations: (1) Pliocene–Early Pleistocene aeolian red clays, (2) the oldest loess–palaeosol sequences, formed from the almost continuous Early Pleistocene dustfall, and (3) the typical, glacial–interglacial loess deposits of the last one million years, without remarkable dust deposition in the warmer periods.
Spatio-temporal distribution of dust storms
Analyses of NASA’s TOMS aerosol measurements have demonstrated that the spatial distribution of dust storms are associated to specific geomorphological environments, while emissions show large seasonal variability related to regional meteorological conditions. Areas with high TOMS AI values are lying in arid-semiarid regions, mostly in geomorphological depressions or at flanks of high mountains. The fine-grained dust material of the sources is the product of former fluvial or lacustrine sedimentary environments. Most of the major sources are situated on remnants of large Pleistocene pluvial lakes, which are nowadays almost totally dried up. The hardened surface of ancient lakebeds can disrupt by bombardment of larger particles, and so the barren deep alluvial deposits are very susceptible to wind erosion. Playas, sabkhas, pans and other ephemeral salt lakes and flats bordered by large sand seas are the largest and most persistent dust source areas of our planet.
Aeolian dust deposits can be considered as one of the most important archives of the past climatic changes. Alternating loess and paleosol strata display the variations of the dust load in the Pleistocene atmosphere. By using the observations of recent dust storms, we are able to employ the Late Pleistocene stratigraphic datasets (with accurate chronological framework) and the detailed granulometric data for making conclusions on the atmospheric dust load in the past.
Age-depths models, created from the absolute age data and stratigraphy, allow us to calculate sedimentation rates and dust fluxes, while grain-size specifies the dry deposition velocity, i.e. the atmospheric residence time of mineral particles. Thus, the dust concentration can be expressed as the quotient of the dust flux and gravitational settling velocity.
Recent observations helped to clarify the mechanisms behind aeolian sedimentation and the physical background of this process has nowadays been well-established. Based on these two main contrasting sedimentary modes of dust transport and deposition can be recognized: the short suspension episodes of coarse (silt and very fine sand) fraction and the long-range transport of fine (clay and fine silt) component. Using parametric curve-fitting the basic statistical properties of these two sediment populations can be revealed for Pleistocene aeolian dust deposits, as it has been done for loess in Hungary. As we do not have adequate information on the magnitude and frequency of the Pleistocene dust storms, conclusions could only be made on the magnitude of continuous background dust load. The dust concentration can be set in the range between 1100 and 2750 μg/m3. These values are mostly higher than modern dust concentrations, even in the arid regions. Another interesting proxy of past atmospheric conditions could be the visibility, being proportional to the dust concentration. According to the known empirical dust concentration–visibility equations, its value is around 6.5 to 26 kilometres.
According to the studies of red clay–loess–palaeosol sequences from China, Central Asia, Alaska, South America and Central Europe, the mineral dust deposition has shown similar pattern since the Pliocene. The typical loess of the last one million years reflects glacial–interglacial conditions, whereas the old loess–palaeosol sequences could be the product of shorter arid–humid climate cycles. The similarity of the sedimentary structures could be caused by the similar global climate conditions, controlled by orbital forcing.
The investigated sections from Hungary (Central Europe) have been affected by local and regional geomorphological and climate factors. Even so, they can be correlated fairly well with the major global climate changes. The Hungarian aeolian dust deposits consist of three main groups of sedimentary formations: (1) Pliocene–Early Pleistocene aeolian red clays, (2) the oldest loess–palaeosol sequences, formed from the almost continuous Early Pleistocene dustfall, and (3) the typical, glacial–interglacial loess deposits of the last one million years, without remarkable dust deposition in the warmer periods.
Spatiotemporal patterns of Saharan dust outbreaks in the Mediterranean Basin
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.
Analyses of NASA’s TOMS aerosol measurements have demonstrated that the spatial distribution of dust storms are associated to specific geomorphological environments, while emissions show large seasonal variability related to regional meteorological conditions. Areas with high TOMS AI values are lying in arid-semiarid regions, mostly in geomorphological depressions or at flanks of high mountains. The fine-grained dust material of the sources is the product of former fluvial or lacustrine sedimentary environments. Most of the major sources are situated on remnants of large Pleistocene pluvial lakes, which are nowadays almost totally dried up. The hardened surface of ancient lakebeds can disrupt by bombardment of larger particles, and so the barren deep alluvial deposits are very susceptible to wind erosion. Playas, sabkhas, pans and other ephemeral salt lakes and flats bordered by large sand seas are the largest and most persistent dust source areas of our planet.
Late Pleistocene variations of the background aeolian dust concentration in the Carpathian Basin: an estimate using decomposition of grain-size distribution curves of loess deposits
Aeolian dust deposits can be considered as one of the most important archives of the past climatic changes. Alternating loess and paleosol strata display the variations of the dust load in the Pleistocene atmosphere. By using the observations of recent dust storms, we are able to employ the Late Pleistocene stratigraphic datasets (with accurate chronological framework) and the detailed granulometric data for making conclusions on the atmospheric dust load in the past.
Age-depths models, created from the absolute age data and stratigraphy, allow us to calculate sedimentation rates and dust fluxes, while grain-size specifies the dry deposition velocity, i.e. the atmospheric residence time of mineral particles. Thus, the dust concentration can be expressed as the quotient of the dust flux and gravitational settling velocity.
Recent observations helped to clarify the mechanisms behind aeolian sedimentation and the physical background of this process has nowadays been well-established. Based on these two main contrasting sedimentary modes of dust transport and deposition can be recognized: the short suspension episodes of coarse (silt and very fine sand) fraction and the long-range transport of fine (clay and fine silt) component. Using parametric curve-fitting the basic statistical properties of these two sediment populations can be revealed for Pleistocene aeolian dust deposits, as it has been done for loess in Hungary. As we do not have adequate information on the magnitude and frequency of the Pleistocene dust storms, conclusions could only be made on the magnitude of continuous background dust load. The dust concentration can be set in the range between 1100 and 2750 μg/m3. These values are mostly higher than modern dust concentrations, even in the arid regions. Another interesting proxy of past atmospheric conditions could be the visibility, being proportional to the dust concentration. According to the known empirical dust concentration–visibility equations, its value is around 6.5 to 26 kilometres.
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