230Th/U age and paleobotanic description of the organic-rich layer from the “Ilya Prorok” section on the Bolshaya Kosha River (Upper Volga basin)

Abstract

This article presents new data from geochronological study the lake-bog organic-rich layer from of the “Ilya Prorok” sequence (Upper Volga Basin, Tver Region) that is a one of the reference sections located on the Russian Plain. Ideas concerning the age and climatic conditions of these organic-mineral deposits formation changed significantly over the long history of their study. As a result, the viewpoint about the belonging of this layer to the Likhvin time began to prevail. However, it is still unclear with which marine isotope-oxygen stage (MIS) the Likhvin interglacial can be compared. According to the paleobotanical study of the lake-bog from the Ilya Prorok section, it was confirmed that the lower part of the lake-bog layer can be attributed to the warm phases of the Likhvin interglacial, while its upper half was deposited during the transition to cooling after the interglacial period. For the first time, the radioisotope age of the buried lake-bog deposits of the reference section “Ilya Prorok” have obtained. According to the data of direct 230Th/U dating, the formation of the sequence probably began no earlier than ~ 326–260 Kyr ago. The 230Th/U age of the buried peat bog from its upper half, according to the results of applying the isochronous approximation, can be correlated with the interval ~256–225 Kyr ago. According to these data, the Likhvin deposits were formed no earlier than MIS-9.

Palaeoenvironmental Conditions of the Upper Middle Pleistocene Warm Intervals in the Upper Volga Region, Northwestern Russia, Based on Palynological, Paleocarpological and Quantitative Geochronological Data

“…Razina. The results were published in [12]. For the buried lacustrine deposits in the lower part of the section (depth 8.2-10.6 m, materials of 2001), the LPAZ sequence was obtained, similar to the PAZ sequence of the Likhvin Interglacial, identified earlier in the same location by V.P.…”

“…Kolesnikova [24] provides a generalised list of macrofossils of predominantly eurythermal wetland plants. Such aquatic plant species as Najas flexilis and the far-eastern species Potamogeton oxyphyllus, which in subsequent studies were identified as the extinct species Caulinia goretsqyi and P. sarjanensis, respectively [12,25], were indicated in this study. Seeds of spruce Picea abies and megaspores of Selaginella selaginoides were identified.…”

“…Palynological data characterise a relatively slight warming of interstadial rank, with dominance of pine forests, separated from the interglacial by cooling with sharp predominance of birch forests with small participation of spruce, the presence of cryophilic shrubs (Betula nana, Alnus fruticosa) and the expansion of open herbaceous communities of Artemisia, Chenopodiaceae and Poaceae. This cooling was attributed to the transition phase between the Likhvin interglaciation and the Dnieper glaciation [12]. In the pollen diagram of the Bolshaya Kosha section studied by V.P.…”

“…Grichuk [23]. The pollen data characterised the interglacial, including its climatic optimum (8.5-9.0 m depth) [12]. In the corresponding LPAZ (L-3), relatively high pollen contents of hornbeam (up to 15% of the terrestrial pollen sum) and fir (up to 12%) were noted.…”

“…Particular attention was drawn to the thick unit of lake/marsh sediments, in which a number of researchers identified layers of the Likhvin epoch, as well as transitional layers to the subsequent glacial part of the 100 ka climatic rhythm. Recently, 230 Th/U dates related to MIS 8 were published for organogenic deposits of this section [12]. We tried to clarify the geochronology of the section by IRSL dating and also carried out a paleocarpological analysis of the upper part of the organogenic sequence, which had not previously been studied by this method, in order to clarify its position relative to the Likhvin Interglacial.…”