Features of silt microstructure formation at the sedimentogenesis stage in the White Sea
The Holocene littoral clay sediments in the White Sea, i.e., organomineral bottom silts, were the objects of study. The work was aimed at investigation of formation and subsequent transformation of deposit microstructure at the initial stages of sedimentation from the viewpoint of current ideas in the physicochemical theory of forming natural fine-grained systems. Silt was sampled in intervals (0-5, 5-10, 10- 15, 15-20, 20-25, 25-30, 30-35, 35-40 cm) from the ship using the sampling thief and dredger “Ocean-0.025” to preserve the natural structure of samples. Silt was prepared for microstructural SEM investigation by the method of vacuum cryogenic processing. The following physicochemical parameters were controlled in samples: moisture (natural and air-dry moisture, moisture at plastic and liquid limits), plasticity index, liquidity index, porosity, porosity coefficient, and the organic matter content. Microstructural studies (qualitative and quantitative analyses of the White Sea silt microstructure) were performed with scanning electron microscope (SEM) LEO 1450 VP. Qualitatively, the silt microstructure was analyzed by SEM images at two magnifications (x250-500 and x4000-8000). The quantitative analysis was performed with “STIMAN 3D” software using a series of 8 SEM images at magnifications х250, х500, х1000, х2000, х4000, х8000, х16000, х32000. The following regularities were revealed from the top to the bottom in the sediment profile: a decrease in the physicochemical indices (W- from 195 to 140% and I<sub>t</sub> from 5.44 to 1.65), a regular increase in density (from 1.18 to 1.35 g/cm<sup>3</sup>) and a decrease in porosity from 88 to 79% as well as in the organic substance content С<sub>org</sub> (from 7.7 to 2.9%); with the particle-size distribution remaining virtually constant. The micromorphology of solid structural units and silt mineral skeleton was qualitatively described. Quantitative estimation permitted obtaining the curves of pore distribution by the equivalent diameters, areas, total areas and shape factor. Inter-particle ultramicropores (0.1-1.0 µm); fine and coarse intermicroaggregate micropores (1.0-10 and 10-100 µm) were distinguished. Proceeding from the quantitative values of integral microstructural indices (grain-size parameter D and anisotropy parameter A), the studied clay sediments were classified by the microstructure types. The bulk of investigated samples were referred to the fine-grained class, weakly-ordered subclass, and coagulation microstructure group. The performed research proves that even within a very narrow depth interval (0.3-50 cm), upon insignificant compaction of newly deposited silt in the White Sea, the features of sedimentogenesis of organomineral sediments are already manifested in the transformation of physicochemical properties and the pore space fabrics. This transformation implies a decrease in the general porosity and the maximal size of micropores, an increase in the amount of fine intermicroaggregate micropores at the expense of the reducing large intermicroaggregate micropores.