Carbon isotope evidence for widespread methane seeps in the ca. 635 Ma Doushantuo cap carbonate in south China

Abstract

Distinctive sedimentary structures, textures, and extremely negative δ 13 C carb values from the Ediacaran Doushantuo cap carbonate (ca. 635 Ma) in south China were taken as evidence for a methane hydrate destabilization event, but existing data for a methane-derived δ 13 C carb signature were reported from a single locality in the Yangtze Gorges area. Here we report negative δ 13 C carb values as low as −48‰ (Vienna Peedee belemnite) from two additional sections ~6 km and 55 km from the original locality. These negative δ 13 C carb values were obtained from isopachous cements that fill stromatactis-like cavities, sheet cracks, and fractures, and from partially recrystallized carbonate crusts, consistent with carbonate precipitation at cold methane seeps. The new data support a widespread methane release event following the Nantuo glaciation in south China (ca. 635 Ma), which may have contributed to the origin of unusual sedimentary and isotope features of cap carbonates.