Problems of 14C dating in fossil soils within tectonically active highlands of Russian Altai in the chronological context of the late Pleistocene megafloods

• Published in: Catena (Giessen) 2020-12, Vol.195, p.104764, Article 104764
• Main Authors: Agatova, A.R., Nepop, R.K., Bronnikova, M.A., Zhdanova, A.N., Moska, P., Zazovskaya, E.P., Khazina, I.V.
• Format: Article
• Language: English
• Subjects: Apparent14C ages; Chuya basin; Late Pleistocene ice-dammed lakes; Megafloods; OSL dating; Paleosols; Radiocarbon dating; Russian Altai
• ISSN: 0341-8162; 1872-6887
• DOI: 10.1016/j.catena.2020.104764

•For the first-time evidence of the Late Pleistocene pedogenesis is presented.•14C date 35870 ± 490 BP (Beta 159972) and reported ages of paleosol are apparent ones.•OSL age 37.0 ± 3.1 ka predates late Pleistocene pedogenesis and the last megaflood.•Redeposited ancient sediments preserve main mineralogical, paleontological features. A complicated tectonic and sedimentation history of the high-mountainous south-eastern part of Russian Altai predetermined the presence of sources of ancient organic matter in the region. During the Pleistocene these deposits were affected by various exogenous processes that leads to mixing of young and old carbon in a unique (for each sample) ratio and strongly affects the results of radiocarbon dating. This paper presents the results of multidisciplinary investigations of pedosedimentary sequences and discusses 14C dating of heterochronous organic matter in paleosols of two locations within the Chuya basin in the chronological context of the late Pleistocene megafloods. In the Kamsug valley reasonable evidence of the oldest (Pleistocene) paleosols in the SE Altai is described for the first time. The mineral composition of sediments and the results of pollen analysis argue for a short-distant redeposition of the Paleogene-Neogene parental substrate. Later it was transformed by Pleistocene pedogenesis: cryogenic processes, particularly solifluction, redox fluctuations resulted in redistribution of Fe and Mn. The analysis of available radiocarbon dates indicates that the contamination of the samples by extraneous carbon and the insufficient data about the features of organic matter of the studied soils don't allow to draw a definite conclusion about the time of the Pleistocene pedogenesis in the region. The presented OSL age 37.0 ± 3.1 ka (GdTL 3424) predates pedogenesis in this location, as well as the last ice-dammed lake formation in the Chuya basin and its further cataclysmic draining followed by the deposition of cross-bedded gravels and pebbles. In the neighbouring Yamanterek valley radiocarbon dating result for paleosol returns erratic (aged) date due to the influx of ancient organic matter. Similarly, this date cannot serve as the upper chronological limit of ice-dammed lakes existence in the Chuya depression. The radiocarbon ages (9.7–11.3 ka cal BP) of charcoals from the underlying lens of humus sands postdate the last ice-dammed lake with the water level at least 1980 m a.s.l. It was demonstrated that short-distant re