The 238U/235U Ratio as an Indicator of Redox Conditions in the Ediacaran Paleobasin (Sequence of the Chaya River, Baikal–Patom Highland, Southern Part of Central Siberia)

Abstract

Abstract—This article is devoted to a study of variations in the U isotopic composition (²³⁸U/²³⁵U) in the section of clastic–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, 2SD) MC-ICP-MS with a ²³³U–²³⁶U double isotope spike. The total variation range of δ²³⁸U in the studied carbonate rocks was −0.91 to −0.01‰. Extremely low values of δ²³⁸U (−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 δ²³⁸U 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.