238U/235U isotopic ratio as redox conditions marker in ediacarian paleobasin: section in Chaya river, Baikal-Patom highland, south of middle Siberia

Abstract

This article is devoted to a study of variations in the U isotopic composition (238U/235U) in the section of clas- tic-carbonate Ediacaran sediments exposed along the Chaya River in the southern part of Central Siberia (Russia). Measurements of 238U/235U in the rocks were performed using a high-precision (±0.07%o, 2SD) MC-ICP- MS with a 233U-236U double isotope spike. The total variation range of S238U in the studied carbonate rocks was -0.91 to -0.01%. Extremely low values of S238U (-0.91%, -0.9%, and -0.84%), which were found in a number of samples from the lower part of the section, are interpreted as the result of postsedimentation processes affecting the rocks. In the overlying sediments, the range of S238U variations is smaller from -0.49 to -0.01%. Here, regular “weighting” of the U isotopic composition observed upwards through the section indicates an increase in the reduction conditions at this time (about 550 Ma) in the paleobasin. This conclusion is consistent with the elevated concentrations of U, Mo, and V in the Late Ediacaran sedimentary rocks of the region studied.