The unusual carbon biogeochemical cycling that occurred in the Ediacaran (ca. 635–541 Ma) ocean may have been critical for ocean oxygenation and Ediacaran life evolution. However, the triggers of the peculiar Ediacaran carbonate carbon isotope (δ13Ccarb) excursions are not well understood. Because mercury (Hg) has a strong affinity for organic carbon, we measured Hg isotope compositions (δ202Hg and Δ199Hg) from Ediacaran marine sediments of South China and South Australia to better understand the causes of the Ediacaran δ13Ccarb excursions. During two local positive δ13Ccarb excursions in the Doushantuo Formation, gradually decreasing Δ199Hg and increasing δ202Hg trends are primarily ascribed to enhanced terrestrial input. Decreasing δ202Hg in both the Doushantuo and Wonoka Formations during the latter part of the Shuram negative δ13Ccarb excursion demonstrates the significant, and potentially widespread, contribution of upwelling Hg associated with dissolved organic carbon (Hg-DOC) from the open deep ocean. New Hg isotope data also suggest that a large Hg-DOC reservoir may have been present in the open ocean continuously from the early to mid–late Ediacaran Period (ca. 635–551 Ma), buffering ocean oxygenation at that time.