Abstract Secular variation of 87Sr/86Sr in carbonate strata has been widely used in regional and global chemostratigraphic correlations. Typically, diagenesis results in higher 87Sr/86Sr signals relative to their primary composition due to the alteration by Rb-rich fluids and radiogenic decay of 87Rb to 87Sr. Surprisingly, 87Sr/86Sr 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 87Sr/86Sr anomalies are closely coupled with positive eNd(t = 560 Ma) values (up to +4.1). Clear covariations between 87Sr/86Sr, eNd, TOC, δ13Ccarb, δ13Corg, and δ18Ocarb 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 δ13Corg 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 87Sr/86Sr signals have neither temporal nor biogeochemical significance. Sr isotope chemostratigraphy in this particular region may not be a reliable tool for stratigraphic correlations.

中文翻译：

---

小学还是中学？沙特阿拉伯埃迪卡拉纪碳酸盐中锶同位素异常的二分法

摘要 碳酸盐岩地层中87Sr/86Sr的长期变化已广泛应用于区域和全球化学地层对比。通常，由于富含 Rb 的流体的改变和 87Rb 至 87Sr 的放射性衰变，成岩作用导致 87Sr/86Sr 信号相对于其主要成分更高。令人惊讶的是，根据全球汇编，沙特阿拉伯埃迪卡拉纪石灰岩中的 87Sr/86Sr 值（从 0.7029 到 0.7059）明显低于典型的埃迪卡拉纪海水值（主要从 0.7080 到 0.7090）。了解这些异常的起源很重要，因为这些地层中保存着早期的大型化石。本研究对两个假设进行了独立评估。第一个假设显示了一个低温情景，在镁铁质源附近有孤立的海洋或湖泊。第二个假设的特点是高温场景，带有幼年热液的深刻叠加。综合 Sr 和 Nd 同位素数据显示，87Sr/86Sr 异常与正 eNd(t = 560 Ma) 值（高达 +4.1）密切相关。发现了 87Sr/86Sr、eNd、TOC、δ13Ccarb、δ13Corg 和 δ18Ocarb 之间的明显协变。基于多条岩石学、野外和地球化学证据，本研究首选第二种假设（即幼年流体引起的热液蚀变）。寄主沉积物中有机质的热分解可能导致残留有机质的 TOC 丰度越来越低，δ13Corg 信号越来越高。我们认为应该谨慎对待埃迪卡拉纪生物辐射发生在孤立的湖泊环境中的概念。这些非常低的 87Sr/86Sr 信号既没有时间意义也没有生物地球化学意义。该特定区域的 Sr 同位素化学地层学可能不是地层相关性的可靠工具。