Chronostratigraphic Position of the Mikulinian Deposits (Case of the Reference Section Near Nizhnyaya Boyarshchina Village, Smolensk Oblast)

The Mikulino (penultimate) interglacial’s deposits are well distinguished according to palynological, carpological, and other types of analysis. Therefore, they serve as an important stratigraphic benchmark in the upper part of the Quaternary sediment cover. However, no agreement has yet been reached on the extent of this interglacial, and the chronological framework of the deposits attributed to it is discussed in the interval from 15 to 70 thousand years. The main goal of this study is to obtain accurate numerical estimates of the age of individual stages of the Mikulino interglacial based on 230Th/U dating data and paleobotanical study of organic-rich deposits from the known reference section “Nizhnyaya Boyarshchina.” The sequence was chosen due to thickness of organic-rich layer reflecting completely the Last Interglacial phases. An improved geochronological approach was applied. On the basis of experimental radiochemical data, organic-rich layers suitable for the use of 230Th/U isochronous approximation were identified. Detailed comprehensive palynological and carpological studies of the lacustrine-bog strata made it possible to identify 91 taxa of fossil flora (59 + 32) and to reveal the change in the phases of vegetation development during the Mikulino interglacial. As a result, the 230Th/U dating of three pieces of lacustrine-bog deposits corresponding to narrow time intervals of plant formations development at different stages of the Last interglacial was fulfilled. The gyttia layer in the depth range of 3.03–2.89 m, correlated with the second half of the M1 pollen zone, was formed ~130–126 Kyr ago at the end of the transitional stage from glaciation to the beginning of the Mikulino interglacial. The peat layers in the depth interval 1.65–1.83 m were deposited ~110–108 Kyr ago and correspond to the first half of the M5 pollen zone, that is, the beginning of the climatic optimum of the Mikulino interglacial. The upper part of peat deposits at a depth of 1.37–1.19 m was formed ~102–97 Kyr ago and is compared with the pollen zone M6, which reflects the second half of the climatic optimum. The time duration of the vegetation development period corresponding to the M1–M6 interval of the Mikulino interglacial includes the substages MIS-5e, MIS-5d and partially MIS-5с and covers approximately 25–30 thousand years. The results obtained indicate that the applied complex approach is promising for establishing the chronology of the Last Interglacial.

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