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The Ediacaran Shuram excursion (SE) records a global decrease in carbonate carbon isotope (δ¹³C_carb) values from +6‰ down to ca. –10‰, representing the largest δ¹³C_carb negative anomaly in Earth history. While the SE is widely recorded in the upper Doushantuo Formation of South China, it shows highly variable δ¹³C_carb profiles among correlative sections. This inconsistent expression of the SE challenges the conventional view of a homogeneous marine dissolved inorganic carbon (DIC) reservoir. A potential process that could explain δ¹³C_carb variability is local mineralization of isotopically distinct authigenic carbonates near the sediment–water interface during early diagenesis. However, a direct test of such authigenic carbonates is still limited. Here, following a recent study on the SE in an intra-shelf environment, we revisited an outer-shelf section, identified and analyzed μm-scale, syn-depositional …