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Methane has been widely regarded as an important source of greenhouse gas that modulated Earth’s paleoclimate. Notably, it has been proposed that a massive release of methane via clathrate destabilization may have played a critical role during and/or in the immediate aftermath of the Marinoan deglaciation. One key piece of supporting evidence for this hypothesis comes from the isotopically anomalous (δ¹³C_carb < –30 ‰) methane-derived authigenic calcite (MDAC) cements within post-Marinoan cap dolostones in South China. However, the origin of these MDAC cements remains hotly contested, including a clumped isotope study that reinterpreted them as resulting from late hydrothermal fluids. To further evaluate these critical yet controversial cements, a detailed investigation by secondary ion mass spectrometry (SIMS) was conducted. Here we report the largest range of δ¹³C_carb values (–53.1 ‰ to +6.3 …