Study on Outwash of Longriba in West Sichuan Province Based on Cosmogenic Nuclide Profiles

It remains a big challenge in dating the outwash and other rapid accumulated mixed depositions using conventional methods， such as Optically Stimulated Luminescence and 14C， because these depositions are mainly composed of coarse gravels with poor stratification， lacking sand interlayers and organic matters. Therefore， it is urgent and meaningful to develop a method to date such kinds of deposits so that we can analyze their paleoenvironment and compare regional differences. In this paper， we selected an outwash deposition profile at Longriba， located in Hongyuan county， Sichuan Province， eastern Tibetan Plateau and tried to date and analyze it by two Cosmogenic Nuclide Profile dating methods： the Monte Carlo calculator and the expansional regression. Results showed that the age of the outwash was （36.7±4.6） ka or （35.2±3.7） ka， the nuclide inheritance was 11.3+4.0/-4.0 ×104 atmos/g， and the post erosion rate at the surface of the outwash was 0.5+0.3/-0.5 cm/ka. We also analyzed the experimental error using density analysis method and Monte Carlo control experimentations， revealing that the exposure prior to deposition yields exposure ages that are too old and incomplete exposure due to post-depositional shielding yields exposure ages that are too young. In contrast with the ages of boulders by cosmogenic nuclide dating， our results point out that the experimental error is dominated by nuclide inheritance for the outwash. The comparisons of regional climate data indicate that the higher precipitation and glacial melt water resulted in higher transport capacity of the river， increased high sediment flux of the river during the glacial （MIS 3b） to interglacial （MIS 3a） transition， so that the outwash aggradation occurred. The Cosmogenic Nuclide Profile dating method is better to deeply analysis the error source of dating， so that it can be used as an effectively method for accurately dating the rapid accumulated mixed depositions.