Pedo-, magneto- and rock magnetic stratigraphy for the late Middle-Upper Pleistocene interval of the Sanzhiyka loess-palaeosol sequence in southern Ukraine

The north-western shore of the Black Sea is represented by thick (30-45 m) and stratigraphically complete Quaternary loess-soil sequences. The Sanzhiyka site recorded a continuous sediment accumulation throughout the past 900 ka: each soil unit has a thickness of 2.5 m, whereas loess units are much thinner, with the exception of the 5 m thick Dnipro (S-L2, MIS 6) loess. We present palaeopedological, rock magnetic and palaeomagnetic results of the upper 19 m of the section from the Lower Zavadivka unit (S-S4, MIS 11) to the Holocene.

Each pedocomplex in the section comprises several soils separated by thin loess beds, or by levels from which wedges penetrate downward. According to the soils’ morphogenetic features and micromorphology, the majority can be attributed to those formed under steppe vegetation. However, the Lower Zavadivka (S-S4) soil unit is presumed to have formed in a rather humid climate, as evidenced by occasional clay coatings and indications of strong carbonate leaching. Furthermore, the S-S4 soils are rubified and weakly gleyed. The Upper Zavadivka (S-S3, MIS 9) soil unit comprises four soils (Chernozems and Cambisols), heavily bioturbated in their lower parts. A subangular blocky and crumby microstructure dominate, and gypsum pedofeatures appear. The Bk horizons of each soil are superimposed on the underlying soils; however, thin sections show signs of primary carbonate leaching. The Potyagaylivka (S-S2, MIS 7) soil unit consists of three soils, two upper soils, which are weakly rubified and have pronounced crumby microstructure. Despite the fact that the lower soil was  strongly altered by subsequent pedogenesis (secondary carbonate accumulation), the signs of primary carbonate leaching and chemical weathering are recognisable. The S-S1 (MIS 5) soil unit comprises the lower Chernozem (Kaydaky subunit) and two upper brown soils, similar to Calcisols (Pryluky subunit). The Kaydaky soil is noticeably leached of carbonates, and numerous krotovinas occur in its subsoil. In the Pryluky soils, abundant coprolites and biogenic channels occur. Similar features are characteristic of the soils in the Vytachiv unit (S-L1S1, MIS 3).

Magnetic susceptibility increases significantly in palaeosols, showing initial, 1-2 optimum, and final phases of pedogenesis in each pedocomplex. Variations in the magnetic susceptibility curve are clearly comparable to marine isotope subtages, for instance two peaks in the Upper Zavadivka (S-S3) soil unit and the Potyagaylivka (S-S2) soil unit are correlated with those of the marine substages of MIS 9 (‘a’ and ‘c­-e’) and MIS 7 (‘a-c’ and ‘e’), respectively. Rock magnetic results indicate the presence of magnetite with a contribution of superparamagnetic grains. Palaeomagnetic analyses confirms the deposition of the studied interval during the Brunhes chron. Magnetostratigraphic study of the lower part of the section aiming at the determination of the Matuyama/Brunhes boundary is currently in progress. Issues of the Quaternary stratigraphy and chronology of loess in southern Ukraine are discussed in detail.

The study was supported by the National Research Foundation of Ukraine, grant number 2020.02/0406, and by the National Science Center, Poland, research project no. UMO-2022/01/3/ST10/00033 (V. Bakhmutov).