Comparative 230Th/U and 14C Dating of a Buried Stump Layer (Western Siberia)

Dating of late Pleistocene sediments remains a challenge in Quaternary chronology, due to the inherent limitations of the C-14 and OSL methods. The Th-230/U radioisotope method is theoretically applicable to wood remnants contained within Pleistocene sediments, but few results have been published to date and in some cases, the age data are ambiguous. This paper tests the use of Th-230/U dating of fossil wood remnants dated earlier by radiocarbon method. We analyzed a buried larch trunk from a well-known stump layer in the Lipovka outcrop, located on the Tobol River bank in Western Siberia. The stump layer is preserved in situ. We determined the specific activities of U and Th isotopes in samples of both modern pine and fossil larch and proposed a model for the incorporation and distribution of U and Th in the buried wood during aging. Complications related with the recognition of geochemical closed systems with respect to U did not allow obtaining completely reliable Th-230/U age. Despite this the Th-230/U age obtained for the uppermost heartwood sample and C-14 ages of the same larch trunk and other wood and vegetation remnants gave consistent results. These age data in combination with previously obtained pollen data testify the stump layer formation during the late cooling stage of the Karganian time (MIS-3, Middle Valdai).