Carbon and oxygen isotope chemostratigraphies of the Yangtze platform, South China: Decoding temperature and environmental changes through the Ediacaran
Multicellular animals first appeared on the earth during the Ediacaran period. However, the relationship betweenthe abrupt biological evolution and environmental changes is still ambiguous. In order to examine seawatertemperature and the carbon cycle through the Ediacaran, we analyzed the carbon and oxygen isotopecompositions of carbonate rocks from drill cores from the Three Gorges area, South China. Importantly, thecore samples include the Nantuo tillite, corresponding to the Marinoan glaciation, through the Doushantuoto the lower Dengying Fms. in ascending order.The δ13C profile displays five positive and five negative anomalies (PI-1 to 5 and NI-1 to 5), and the oxygenisotopes display very high absolute values around 0‰ with the highest at +1.83‰. The combined δ18O andδ13C chemostratigraphies display both positive and negative correlations between the δ18O and δ13C values.The occurrence of the negative correlations supports the preservation of primary δ18O and δ13C values.The sample NI-4 has a negative correlation of the δ18O and δ13C excursions. The correlation supports a primarysignature for both δ18O and δ13C variations. The positive δ18O excursion, accompanied by evidence of a eustaticsea-level fall, provides direct evidence for global cooling in the mid-Ediacaran; the 580 Ma GaskiersGlaciation is a potential candidate for this global cooling event. The negative δ13C excursion was possibly causedby an increase in remineralization of dissolved organic carbon (DOC) due to enhanced continentalweathering during the glaciation.Sample NI-5 is characterized by very low δ13C values, down to −10‰, corresponding to the Shuram-Wonoka-Pertatataka Excursion. The cause of the δ13C negative excursion is still not clear. However, a ubiquitousoccurrence in excursions worldwide, and the lower δ13C values in deeper sections favor the enhancementof remineralization and respiration rather than secondary alteration, a restricted sea environmentand lithification in coastal areas.
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