Composition of bottom sediments of Lake Sasyk (Ukraine) based on granulometric analysis[Склад донних відкладів оз. Сасик (Україна) за даними гранулометричного аналізу]Maryna Krochak1 https://orcid.org/0000-0003-4144-7098 Igor Shuraev2 https://orcid.org/0000-0002-6289-8632 Oleksander Paryshev2 https://orcid.org/0000-0003-1318-9650 1 Institute of Geological Sciences, National Academy of Sciences of Ukraine (Kyiv, Ukraine) 2 State Scientific Institution ‘Centre for Problems of Marine Geology, Geoecology, and Sedimentary Ore Formation, NAS of Ukraine’ (Kyiv, Ukraine) Cite as Krochak, M., I. Shuraev, О. Paryshev. 2025. Сomposition of bottom sediments of Lake Sasyk (Ukraine) based on granulometric analysis). GEO&BIO, 27: 121-130. [Ukrainian, with English summary] doi: https://doi.org/10.53452/gb2710 pdf: gb2710_121-130-kro-fin.pdf Abstract The aim of this study was to investigate the structure of sediments in Lake Sasyk (north-western coast of the Black Sea, Odesa Oblast, Ukraine) and the characteristics of their spatial distribution. The research was based on the results of a granulometric analysis of bottom sediment samples. To visualise the granulometric data, mathematical processing of the results was carried out using the Surfer graphical modelling software, and distribution maps of clay, silt, sand, and coarse-grained fractions across the lake’s area were created. The coarse-grained fraction is represented by shell detritus of various sizes and is found only in the western part of the northern section of the lake. An increased sand fraction content was recorded in the northern part of the lake, near the inflows of the Sarata and Kogylnyk rivers, where solid discharge deposition and sediment accumulation occur in an underwater alluvial cone. The predominant bottom sediments of the lake consist of silt-clay and clayey silt. The content of the sandy fraction (grains 0.05–2 mm in size) is minimal and does not exceed a few per cents. The silt fraction (grains 0.005–0.05 mm) varies between 22% and 64%. The saturation of mud deposits with clastic material (sand and silt fractions) decreases towards the central parts of the lake, forming a zonal-concentric distribution pattern typical of lake basins. The distribution of the clay fraction is significantly influenced by the influx of fine-grained clay material transported by Danube waters entering the lake through the Danube–Sasyk Canal in its south-western part. A zone of intense clay sediment accumulation was identified, starting near the canal’s mouth and extending from the south-west to the north-east, reaching the central part of the lake and further north. This pattern of clay fraction transport and deposition is controlled by the prevailing lake currents and contributes to the overall silting of the lake’s water area. Additional electron microscopy and X-ray spectral studies of the sediments revealed a significant concentration of sulphate ions in the near-bottom waters, which is sufficient for gypsum sedimentation. Key words Lake Sasyk, bottom sediments, granulometric analysis, spatial distribution of fractions, silty clays, accumulation, Danube–Sasyk Canal. Correspondence to Maryna Krochak; Institute of Geological Sciences, NAS of Ukraine; 55B Olesia Honchara Street, Kyiv, 01054 Ukraine, Email: mkrochak1960@gmail.com, orcid: 0000-0003-4144-7098. Article info Submitted: 18.04.2025. Revised: 19.06.2025. 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