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Cryogenian-Ediacaran Transition - Organic Carbon Isotope Constraints on the Dissolved Organic Carbon (Doc) Reservoir  

In some Ediacaran successions prominent negative carbonate carbon isotopes (δ13Ccarb) anomalies are accompanied by organic carbon isotopes, that are invariant or decoupled, (δ13Corg) values and have been interpreted as the results of a large dissolved oxygen carbon (DOC) reservoir that is capable of buffering carbon isotopes of organic matter, that has been remineralised. This oceanic DOC reservoir that has been inferred was believed to have initiated at the onset of the glaciations of the Cryogenian about 720 Ma and persisted until the Late Ediacaran less than 500 Ma. Analyses of the carbon isotope of the basal Doushantou Formation, South China, that dated to about 635 Ma, revealed (1) the cap carbonate has (δ13Ccarb) of around -26‰ (VPDB) and a Δδ13C (22 ± 2‰) and (2) and overlying this is an organic-rich black shale and shaly dolostone that has a more negative δ13Corg  (-28‰ to -35‰) and higher Δδ13C (28‰ - 30‰). Within the overlying Doushantou Cap Carbonate an interval that is 4 m thick, a shift of + 6‰ is shown by both δ13Ccarb and δ13Corg. The cap carbonate δ13Corg values are associated with low TOC, most of which are less than 0.1%, the Palaeoceanographic significance of which requires more tests in other basins from the Ediacaran. Following the deposition of cap carbonate the co-varying positive shift in δ13Ccarb and δ13Corg  is, according to the authors1, best interpreted as the result of a rapid increase of burial of organic carbon, and they1 suggest this may have led to the increase of oxygen, which heralded the first appearance of animals in layers a few metres above the Doushantou Cap Carbonate. There doesn't seem to have been a large oceanic DOC reservoir in the ocean of the Early Ediacaran. If there was an excess oceanic DOC that is required to explain the δ13C excursions in the Shuram and Doushantou Formations of the Ediacaran it could only have developed in the Ediacaran following the deposition of the cap carbonate.

Sources & Further reading

  1. Jiang, Ganqing, Xinqiang Wang, Xiaoying Shi, Shihong Zhang, Shuhai Xiao, and Jin Dong. "Organic Carbon Isotope Constraints on the Dissolved Organic Carbon (Doc) Reservoir at the Cryogenian–Ediacaran Transition." Earth and Planetary Science Letters 299, no. 1–2 (10/15/ 2010): 159-68.
Author: M. H. Monroe
Last updated 31/05/2013

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