Organic carbon isotope constraints on the dissolved organic carbon (DOC) reservoir at the Cryogenian–Ediacaran transition

Prominent negative carbonate carbon isotope (δ13Ccarb) anomalies from some Ediacaran successions are accompanied by invariant or decoupled organic carbon isotope (δ13Corg) values and have been interpreted as resulting from the remineralization of a large dissolved organic carbon (DOC) reservoir capable of buffering carbon isotopes of organic matter. This inferred oceanic DOC reservoir was thought to have initiated with the onset of Cryogenian glaciations (ca. 720 Ma) and lasted for millions of years until the late Ediacaran Period (< 560 Ma). Carbon isotope analyses of the basal Doushantuo Formation (ca. 635 Ma) in south China reveal that (1) the cap carbonate has δ13Corg around −26‰ (VPDB) and relatively low Δδ13C (22 ± 2‰) and (2) the overlying organic-rich black shale and shaly dolostone have more negative δ13Corg (−28‰ to −35‰) and higher Δδ13C (28‰–30‰). Both δ13Ccarb and δ13Corg show a + 6‰ shift within a 4-m-thick interval overlying the Doushantuo cap carbonate. The δ13Corg values of the cap carbonate are associated with low TOC (mostly < 0.1%); their paleoceanographic significance requires further tests in other Ediacaran basins. The co-varying positive shift in δ13Ccarb and δ13Corg following cap carbonate deposition is best interpreted as resulting from a rapid increase in organic carbon burial, which may have resulted in the rise of oxygen and heralded the first appearance of animals a few meters above the Doushantuo cap carbonate. The data suggest that a large oceanic DOC reservoir did not exist in the early Ediacaran ocean. Excess oceanic DOC required to explain the Ediacaran Shuram and upper Doushantuo δ13C excursions, if it existed, had to be developed during the Ediacaran Period after cap carbonate deposition.