Granulometric characterization of paleosols in loess series by automated static image analysis
An automated image analysis method is proposed here to study the size and shape of siliciclastic sedimentary particles of paleosols of Central European loess sequences.
New paper: Varga et al. (2018) Sedimentary Geology 370, pp. 1-14. DOI: j.sedgeo.2018.04.001
Several direct and indirect measurement techniques are available for grain size measurements of sedimentary mineral particles. Indirect techniques involve the use of some kind of physical laws, however, all requirements for calculations are in many cases not known. Even so, the direct manual microscopic observation and measurement of large, representative number of grains is time-consuming and sometimes rather subjective. Therefore, automated image analyses techniques provide a new and perspective way to analyse grain size and shape sedimentary particles.
Here we test these indirect and direct techniques and provide new granulometric data of paleosols. Our results demonstrate that grain size data of the mineral dust samples are strongly dependent on shape parameters of particles, and shape heterogeneity was different of the different size classes. Due to the irregular grain shape parameters, uncertainties have arisen also for the sizes.
In this paper we present a possible correction procedure to reduce the differences among the results of the laser diffraction and image analysis methods. By applying new correction factors, results of the two approaches could be get closer but the most definite factor, the unknown thickness of particles remained a problem to solve. The other presented method to assess the uncertain 3rd dimension of particles by their intensity-size relationships makes us able to reduce further the deviations of the two sizing methods.
Interpretation of sedimentary (sub)populations extracted from grain size distributions of Central European loess-paleosol series
Varga, Gy., Újvári, G., Kovács, J. (in press). Interpretation of sedimentary (sub)populations extracted from grain size distributions of Central European loess-paleosol series. Quaternary International
Grain size proxies of aeolian dust deposits have widely been applied in environmental and sedimentary studies. However, large body of research papers are not taking into consideration that a complex grain size distribution curve cannot be an indicator of a single one environmental factor (e.g. wind speed/strength, transportation distance, aridity).
The aim of the present paper is to discuss the main differences of frequently used statistical methods and to provide possible interpretations of the results by applying these various approaches on the high-resolution loess-paleosol profile of Dunaszekcső, South Hungary (Central Europe). Beside single statistical descriptors (mean, median, mode) of grain size and simple indices of size-fraction ratios (U-ratio, Grain Size Index), some more complex algorithms were also used in our paper. The applied parametric curve-fitting, end-member modelling and hierarchical cluster analysis techniques are using the whole spectrum of the measured grain size distributions and provide a more reliable and more representative results even in case of small scale variations.
According to our findings, approaches which provide direct linkage among simple statistical descriptors and single atmospheric or other environmental elements are rather oversimplified as properties aeolian dust deposits are influenced by the integrated effects of several concurrent processes. Differences of more complex decomposition methods arise from the different approach and scope. End-members are determined from the unmixing based on the covariance structure of the whole grain size data-series of the section, while the parametric curve-fitting is based on the one-by-one deconvolution of the grain size distribution curves. End-members of loess-paleosol samples are regarded as representation of the average dust grain size distribution of various temporal sediment clusters of seasonal or other short-term intervals, while (sub)populations by parametric curve-fitting are proposed to illustrate process-related elements of background and dust storm depositional components for each sample. Results of cluster analysis represent similar grouping conditions as end-member modelling with a reduced sedimentary and genetically meaning.
How to fit Weibull functions?
You can separated the different sedimentary sub-populations by using a simple MS Excel software with SOLVER. The difference between the measured and modelled values (sum of two/three Weibull distributions) has to be minimized.
Fuerteventura in widescreen
International Symposium on eolian Dynamics, Paleosols and environmental Change in Drylands
Field impressions in widescreen
15-16 March, 2017: Fuerteventura, Spain
NKFI K120620 - Summaries
Summary of the research and its aims for experts
The proposed research is aimed at to analyse the size and shape properties of individual particles of aeolian dust deposits which are standing in the focal point of environmental studies. The development and application of a new granulometric paleoenvironmental proxy is also proposed in the project. In the course of the research, the loess-paleosoil series of the Carpathian Basin would be investigated which are covering almost the half of the area and are regarded as one of the most important archives of the paleoclimatic and paleoenvironmental changes of the last 1 million years. The scope of the studied deposits would be completed with the analyses of samples collected from recent Saharan dust deposition events and materials from Mediterranean terra rossa soils with a proven, dominant North African dust contribution.
The applied analytical methods of widely used laser diffraction grain size measurements will be supplemented with measurements of size and shape parameters from automated image analysis of pictures taken by optical microscopes. So far similar sedimentological studies based on the quantitative size (e.g. circle-equivalent diameter, length, width, perimeter), shape (e.g. circularity, convexity) parameters, absorbing light-dependent intensity values and chemical composition (from Raman-spectra) of several hundreds of thousands of individual particle have not been carried out. Granulometric analyses augmented with chemical composition determinations would be the basis of a new paleoenvironmental proxy proposed to be developed in the project.
What is the major research question?
In the course of the proposed project, the mechanisms of glacial, loess-forming sedimentary processes and the interglacial, environment-related post-depositional alterations will be reconstructed based on the granulometric parameters of aolian dust deposits. Grain shape and shape distribution of clastic deposits provide information on the type of the transport agent and transport distance, while the size and size distribution parameters are allowing us to reconstruct the power of the transport agent, the dynamics of transport and the distance of possible source area(s).
During the interglacials with different length and intensity, soil forming processes played the dominant role. At the same time, however, the contribution of a minor aeolian dust deposition has to be taken into account also during interglacial periods, which fine-grained dust material is able to modify the mineralogical and physicochemical properties of paleosoils. The post-depositional processes have also been archived in the individual mineral particles. The shape parameters of detrital and secondary particles formed by weathering processes are different. The ratio of distinguished primary and secondary mineral particle-clusters and the minor changes of shape parameters (e.g. surface area of particles derived from convexity values) of primary particles will be compared to geochemical paleoenvironmental proxies to develop a new, independent granulometric paleoenvironmental indicator.
What is the significance of the research?
Preliminary data indicated that routinely applied laser diffraction grain size results and from those derived information on past sedimentary environments are oversimplified. To the application of laser diffraction using Mie-scattering theory, the knowledge of optical properties (refractive index and absorption coefficient) of the sample and the dispersant is needed. However, it is a difficult issue in the case of polymineral material; on the other hand absorption coefficients are also dependent on grain shape parameters. Contrarily, automated image analysis based grain size and shape determinations provide direct information on granulometric properties.
The size, shape, intensity and chemical identity data of the proposed research will be able to revaluate and refine the results of previous grain size measurements of aeolian dust deposits. The method proposed here will allow us to gain insight into the mechanisms of loess formation and into environmental conditions both in a past (loess-paleosoil system) and a recent (dust-soil system) context.
Generally, a spherical shape is supposed during grain size measurements and the results are converted into some kind of equivalent diameter, so these cannot be regarded as precious values. Particle shape determines also the settling velocity of mineral grains modifying so the parameters (e.g. dust concentration assessments from the quotient of dust flux and grain size/shape dependent settling velocity) derived from them.
Grain shape distribution data allow us to determine the ratio of detrital and postdepositional secondary particles. So, the separation of sedimentary and environmental signals is also feasible. The correctness of theoretical, mathematical-statistical detachment of sedimentary subpopulation can also be revised and refined by determination of granulometric clusters based on size and shape parameters obtained from automated image analysis of particles.
Summary and aims of the research for the public
The climate of the Carpathian Basin can be characterized by two major opposite states of long-term climatic regimes; cold-dry glacial and warm-moist interglacial periods have determined the environmental character of the area. During glacial periods due to the increased frequency and intensity of dust storms, aeolian dust deposition became the dominant sedimentary factor and from the accumulated dust material loess was formed. However during the moist and warm interglacial periods the upper part of the loess was formed to soil. These dark layers were buried by dust during the next glacial phase, and archived the warm interglacial period as paleosoil. These cyclic loess-paleosoil series are outstanding archives of climatic changes of the last 1 million years.
Grain size of aeolian dust deposits fall into a very narrow range as a consequence of the extremely selective nature of wind sediment transport. Diameters of windblown particles are generally between some micrometres to hundred micrometres. The continuously developing measurement technology makes possible not only the high-precious determination of grain size, but the deeper understanding of size and shape distribution parameters of several tens or hundreds of thousands of individual particles. Based granulometric parameters of this new procedure, the development of a new environmental indicator and the reconstruction of sedimentary mechanisms and environment-related postdepositional alterations are also possible.
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