The Ediacaran Period (ca. 635–541 Ma) witnessed the earliest paleontological evidence for macroscopic animals (i.e., Ediacara biota) and geochemical observations of the largest carbon cycle anomaly in Earth history (i.e., Shuram Excursion, SE). Numerous hypotheses have been proposed for the origins of the SE, ranging from primary seawater anomaly to syn- or post-depositional diagenesis. Despite intensive geochemical and theoretical work published in the past decade, empirical evidence that is strictly based on fundamental petrographic results at the micrometer scale is still limited. To evaluate depositional compositions and diagenetic effects on samples from the SE, we investigated the EN3 interval in the Doushantuo Formation of South China via integrated cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), and scanning electron microscope (SEM). Detailed petrographic observations reveal that the EN3 limestone is dominated by calcite microspar, with minor but variable amounts of disseminated zoned dolomite crystals. The former likely formed via neomorphism of depositional micrite, while the latter was the result of progressive post-depositional dolomitization. The mean values of paired SIMS δ¹³C_calcite and δ¹³C_dolomite compositions are indistinguishable in each sample and consistent with published micro-drilled bulk-powder δ¹³C values, which we interpret to represent depositional “background” signals of seawater dissolved inorganic carbon (DIC). We also observed μm-scale variability of SIMS δ¹³C data that may reflect a variable diagenetic overprint after deposition. Our integrated petrographic and geochemical results are consistent with a depositional origin of the SE and provide little evidence for the hypothesized isotope alteration by meteoric and mixing-zone diagenesis or late burial diagenesis. In light of this study, we propose that the SE indeed represents a marine carbon cycle anomaly that bears a close temporal link to the Ediacaran surface environment.

埃迪卡拉纪（约 635-541 Ma）见证了宏观动物（即埃迪卡拉生物群）的最早古生物学证据和地球历史上最大的碳循环异常（即 Shuram Excursion，SE）的地球化学观测。已经提出了许多关于 SE 起源的假设，范围从原始海水异常到同沉积或沉积后成岩作用。尽管过去十年发表了大量的地球化学和理论工作，但严格基于微米尺度基本岩相学结果的经验证据仍然有限。为了评估沉积成分和对东南部样品的成岩作用，我们通过综合阴极发光 (CL)、二次离子质谱 (SIMS)、和扫描电子显微镜（SEM）。详细的岩相观察表明，EN3 石灰岩以方解石微晶石为主，具有少量但数量不等的浸染分带白云石晶体。前者可能通过沉积泥晶的新形貌形成，而后者是沉积后白云石化作用的结果。配对 SIMS δ 的平均值¹³ C_方解石和 δ ¹³ C_白云石成分在每个样品中无法区分，并且与公布的微钻散装粉末 δ ¹³ C 值一致，我们将其解释为代表海水溶解无机碳 (DIC) 的沉积“背景”信号。我们还观察到 SIMS δ ^{13 的}微米级变异性C 数据可能反映沉积后可变的成岩叠印。我们综合的岩石学和地球化学结果与东南部的沉积起源一致，并且几乎没有为由陨石和混合带成岩作用或晚期埋藏成岩作用引起的假设同位素改变提供证据。根据这项研究，我们提出 SE 确实代表了一种海洋碳循环异常，它与埃迪卡拉纪表面环境有着密切的时间联系。