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Palaeoenvironmental and palaeoclimatic inferences based on X-ray computer tomography: a case study of alkaline lake deposits in Hungary

Alzoubi Nour N., Gulyas Sandor, Geiger Janos

Geologos

2023

Vol. 29, No. 1

33--49

Widely distributed freshwater carbonate sediments, i.e., limestone, dolomitic limestone and dolomite, developed in inter-dune alkaline ponds of the Danube-Tisza Interfluve in the centre of the Carpathian Basin during the Holocene. The key parameters that determine the formation of any given type of carbonate mineral (calcite, dolomite) are temperature, evaporation rate, pH and ion concentrations, in addition to CO2 absorption by aquatic plants. CT analysis is capable of recording small-scale density variations attributable to compositional differences of sedimentary rocks. As the type and proportion of rock-forming minerals and other components is an artifact of past environmental and climatic conditions, CT values may act as potential palaeoenvironmental proxies. The present study compares variations in rock-forming components obtained for freshwater carbonates utilizing the CT method with already available geochemical and palaeoecological proxy data. Variations in molluscan ecology and isotope geochemistry, sedimentation times and CT-based rock density values all indicate the relevance of millennial-scale, climate-driven changes in carbonate formation. As previously observed, the emergence of colder conditions in the North Atlantic, which resulted in increased cyclonic activity and heavier rainfall in western Europe and the Danube watershed area between 10.3 and 9.3 kyr cal BP, resulted in the emergence of humid conditions favouring a rise in the groundwater table at our site and precipitation of calcite from pore waters as opposed to high-magnesium calcite. This is clearly reflected in a negative shift in CT density values in our dated rock samples.

Defining rock-forming components of Holocene freshwater carbonates via univariate statistical and mixture analysis of computer tomography data

Alzoubi Nour Nayef Hassan, Geiger Janos, Gulyas Sandor

Studia Quaternaria

2022

Vol. 39, Nr 2

113--128

Carbonate rocks are among the sedimentary systems which preserve information on the formation and diagenetic history expressed in its composition (distribution of its major rock-forming components (RFC). For estimating RFC proportions at the micro-scale, a simple counting of visible RFCs in thin sections using overlaid grids is a long-used, well-established technique. However, computer tomography (CT) analysis provides us with quantitative data in 3D at both the scale of the entire sample and a resolution defined by dimensions of the voxels at the micro-scale. The quantitative data expressed in Hounsfield units (HU) correlates with the density of RFCs. In this work statistical properties of CT-based data for selected freshwater carbonate samples from the Danube-Tisza Interfluve have been assessed using histograms and boxplots. Univariate statistical parameters characterize each sample. The maximum-likelihood method of mixture analysis has been adapted to recover and estimate the parameters of these subpopulations. Subpopulations have been defined in the form of overlapping intervals using statistical parameters gained (mean±2STD). Five major components have been defined: empty and partially or entirely filled pores by calcite, limestone micrite, dolomite micrite matrix and limonite saturated matrix.