中国科学院南京地质古生物研究所机构知识库

Secular variation of Sr-87/Sr-86 in carbonate strata has been widely used in regional and global chemostratigraphic correlations. Typically, diagenesis results in higher Sr-87/Sr-86 signals relative to their primary composition due to the alteration by Rb-rich fluids and radiogenic decay of Rb-87 to Sr-87. Surprisingly, Sr-87/Sr-86 values in the Ediacaran limestones from Saudi Arabia (from 0.7029 to 0.7059) are significantly lower than typical Ediacaran seawater values (mostly from 0.7080 to 0.7090) based on a global compilation. Understanding the origin of these anomalies is important insofar as early macrofossils are preserved in these strata. Two hypotheses have been independently evaluated in this study. The first hypothesis shows a low temperature scenario with isolated oceans or lakes in proximity to a mafic source. The second hypothesis is characterized by a high temperature scenario with profound overprints by juvenile hydrothermal fluids. Integrated Sr and Nd isotope data reveal that the Sr-87/Sr-86 anomalies are closely coupled with positive epsilon Nd(t = 560 Ma) values (up to +4.1). Clear covariations between Sr-87/Sr-86, epsilon Nd, TOC, delta C-13(carb), delta C-13(org), and delta O-13(carb) were found. Based on multiple lines of petrographic, field, and geochemical evidence, the second hypothesis (i.e., hydrothermal alteration by juvenile fluids) is preferred in this study. Thermogenic breakdown of organic matter in host sediments may have caused the increasingly low TOC abundance and high delta C-13(org) signals of the residual organic matter. We argue that the concept that the Ediacaran biotic radiation took place in an isolated lake environment should be treated with caution. These remarkably low Sr-87/Sr-86 signals have neither temporal nor biogeochemical significance. Sr isotope chemostratigraphy in this particular region may not be a reliable tool for stratigraphic correlations.