Multicellular animals first appeared on the earth during the Ediacaran period. However, the relationship between the abrupt biological evolution and environmental changes is still ambiguous. In order to examine seawater temperature and the carbon cycle through the Ediacaran, we analyzed the carbon and oxygen isotope compositions of carbonate rocks from drill cores from the Three Gorges area, South China. Importantly, the core samples include the Nantuo tillite, corresponding to the Marinoan glaciation, through the Doushantuo to the lower Dengying Fms. in ascending order.
The delta C-13 profile displays five positive and five negative anomalies (PI-1 to 5 and NI-1 to 5), and the oxygen isotopes display very high absolute values around 0(parts per thousand) with the highest at + 1.83 parts per thousand. The combined delta O-18 and delta C-13 chemostratigraphies display both positive and negative correlations between the delta(18)o and delta C-13 values. The occurrence of the negative correlations supports the preservation of primary delta O-18 and delta C-13 values.
The sample NI-4 has a negative correlation of the delta O-18 and delta C-13 excursions. The correlation supports a primary signature for both delta O-18 and delta C-13 variations. The positive delta O-18 excursion, accompanied by evidence of a eustatic sea-level fall, provides direct evidence for global cooling in the mid-Ediacaran; the 580 Ma Gaskiers Glaciation is a potential candidate for this global cooling event. The negative delta C-13 excursion was possibly caused by an increase in remineralization of dissolved organic carbon (DOC) due to enhanced continental weathering during the glaciation.
Sample NI-5 is characterized by very low delta C-13 values, down to -10 parts per thousand, corresponding to the Shuram-Wonoka-Pertatataka Excursion. The cause of the delta C-13 negative excursion is still not clear. However, a ubiquitous occurrence in excursions worldwide, and the lower delta C-13 values in deeper sections favor the enhancement of remineralization and respiration rather than secondary alteration, a restricted sea environment and lithification in coastal areas.

delta C-13 chemostratigraphies; delta O-18 chemostratigraphies; Doushantuo Fm.; The Ediacaran; Shuram excursion; Gaskiers Glaciation

Northwest University; Chang'an University