High primary productivity during the Ediacaran Period revealed by the covariation of paired C-isotopic records from South China

Abstract

Remarkable carbonate carbon isotope (δ13Ccarb) excursions accompanied by invariant organic carbon isotope (δ13Corg) values have been observed in many late Neoproterozoic successions. This decoupled relationship between δ13Ccarb and δ13Corg in the Precambrian differs from most Phanerozoic records which have been substantially imprinted by aqueous photosynthesis, and it has been tentatively interpreted as resulting from exotic organic matter inputs or diagenesis. In this study, we present high-resolution δ13Corg records in combination with previously published δ13Ccarb data from a continuous drill core from the Yangtze platform of South China. The studied interval spans the Cryogenian Nantuo Formation, the Ediacaran Doushantuo and Dengying formations, and the base of the Cambrian Yanjiahe Formation. The δ13Ccarb profile of this drill core can be reliably correlated with contemporaneous sequences and it reveals multiple negative δ13Ccarb excursions associated with phosphorite deposits. Intriguingly, the δ13Corg profile exhibits a co-varying trend with the δ13Ccarb profile, and the coupled relationship between δ13Ccarb and δ13Corg is rare in geological records of the Precambrian. The δ13Corg values of the drill core vary from −22.13‰ to −34.12‰, and the overall isotopic difference (Δ13Ccarb-org) between δ13Ccarb and δ13Corg ranges between 19.61‰ and 34.58‰. Most of the δ13Corg and Δ13Ccarb-org data can be interpreted in terms of oxygenic photosynthesis, probably mainly by eukaryotic algae. However, it is possible that exotic organic matter, most likely DOC from the deep ocean, may also have contributed to the sedimentary organic matter composition of specific intervals studied herein. Moreover, high primary productivity in shallow water may have been sustained for a long period in phosphate-rich conditions, and the organic matter generated by photosynthesis may have been quantitatively sufficient to overwhelm the DOC buffering effect in the total δ13Corg signal throughout the Ediacaran Period. The enhanced burial of organic matter in sediments would gradually result in the accumulation of oxygen, both in the ocean and atmosphere, which would likely have paved the way for macroscopic animal evolution.