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

Abstract

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 redeposition of the Paleogene-Neogene sediments in the region preserves their main mineralogical and paleontological features. It gives an opportunity to recognise such redeposited sediments which contain geological carbon. Meanwhile the redeposition followed by pedogenesis introduces younger organic matter. Diagenesis of such originally different organic matter contributes the final pool of organic carbon and results in the formation of heterogenous and heterochronous organic matter. This should be taken into account while 14C dating and interpreting the obtained results.