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Radiocarbon dating of organic-rich deposits: Difficulties of paleogeographical interpretations in highlands of Russian Altai

Nepop R.K., Agatova A.R., Bronnikova M.A., Zazovskaya E.P., Ovchinnikov I.YU., Moska P.

Geochronometria

2020

Vol. 47, no.1

138--153

The high mountainous southeastern part of Russian Altai is characterized by complicated sedimentation history. As a result of tectonic movements, Paleogene, Neogene, and even more old Carboniferous and Jurassic organicrich deposits had been partly uplifted and exhumed on the ridge’s slopes, where during the Pleistocene, they were affected by various exogenous processes including glaciation, glacio-fluvial erosion, winnowing activity of ice-dammed lakes, sliding during lake-draining events, followed by further intensive Holocene erosion, pedogenesis, and permafrost formation/degradation. Remobilized ancient organic matter had been involved into geomorphic and pedogenesis processes and affected the results of radiocarbon dating. Numerous radiocarbon ages obtained revealed several typical problems in interpretation of dating results, which was confirmed by multidisciplinary investigations of associated sediments in a wider regional context. This article presents a discussion on obtained apparent radiocarbon dates of organic material from ten sections of the SE Altai. In addition to radiocarbon analysis, in each case multidisciplinary study was carried out in order to properly interpret obtained dates, as well as to explain the inability of directly using apparent 14C ages as a geochronological basis for paleogeographical reconstruction. The analysis presented is of vital importance for establishing the chronology of formation of large ice-dammed lakes and their cataclysmic draining; revealing chronology and paleoenvironmental conditions of pedogenesis in the highlands of the SE Altai; and estimating the range and magnitude of the tectonically driven topography rebuilding in the post-Neogene time.

Pleistocene glaciations of the SE Altai, Russia, based on geomorphological data and absolute dating of glacial deposits in Chagan reference section

Agatova A. R., Nepop R. K.

Geochronometria

2017

Vol. 44, no. 1

49--65

Geomorphological evidence of at least two Pleistocene glacial epochsis noted within the Chagan-Uzun river basin, SE Altai. A review and analysis of all available absolute dates for reference Chagan section is presented. The highest correlation amongst all TL dates is observed for the lens of glacio-lacustrine sediments – the most suitable among glacial deposits for luminescence dating, and indicates its possible Middle Pleistocene age. IRSL dates obtained from feldspar indicate a Middle Pleistocene age of moraines already in the upper part of the section. The small number of obtained IRSL dates does not allow making geochronological reconstructions of the Pleistocene glaciations, but gives the possibility for further experiments with different variation of OSL (IRSL) techniques. Strong low temperature peak in TL signal and strong response to IR stimulation are specific regional quartz features, which could be explained by combination of short transportation distance and low number of depositional cycles for mineral grains. Available radiocarbon dates of carbonate concretions from this section are not related to the age of moraine sedimentation and most likely indicate the period of the Chagan river incision into the ancient glacial deposits. This study has shown that TL method is not valid for dating glacial sediments and TL dates cannot be utilized as chronostratigraphic markers. Generally, the Chagan section could hardly serve as a reference section for the Altai stratigraphy; available depositional correlation schemes for the Russian Altai and Siberia which are based on several TL dates obtained in the last century needs to be improved.