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

¹ Faculty of Earth Sciences, Key Laboratory of Biogeology and Environmental Geology of the Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China, and Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
² Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA
³ Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
⁴ Faculty of Earth Sciences, Key Laboratory of Biogeology and Environmental Geology of the Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China
⁵ Faculty of Material Science and Chemical Engineering, China University of Geosciences, Wuhan, Hubei 430074, China

Correspondence: *E-mails: js-wang{at}cug.edu.cn; Ganqing.Jiang{at}unlv.edu; xiao{at}vt.edu.

Distinctive sedimentary structures, textures, and extremely negative ¹³C_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 ¹³C_carb signature were reported from a single locality in the Yangtze Gorges area. Here we report negative ¹³C_carb values as low as –48 (Vienna Peedee belemnite) from two additional sections ~6 km and 55 km from the original locality. These negative ¹³C_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.

Key Words: carbon isotopes • methane seeps • Ediacaran • Doushantuo cap carbonate • south China