EDIACARAN–EARLY CAMBRIAN CARBON CYCLES AND OCEAN OXYGENATION EVENTS

The Ediacaran–Early Cambrian interval (ca. 635-520 Ma) is characterized by prominent negative δ¹³C excursions and ocean oxygenation events. The link between these events has been ascribed to oxidation of an unusually large dissolved organic carbon (DOC) reservoir that was sufficient to buffer δ¹³C_org and result in decoupled δ¹³C_carb and δ¹³C_org. Recent studies, however, revealed large temporal δ¹³C_org fluctuations and coupled δ¹³C_carb–δ¹³C_org at multiple Ediacaran and Early Cambrian intervals, challenging a long-lasting and progressively decreasing DOC reservoir that was thought to have initiated since ca. 720 Ma. To better understand the casual link between negative δ¹³C excursions and ocean redox evolution, new and existing paired δ¹³C_carb and δ¹³C_org data from South China are compiled and compared with geochemical data for oxygenation events. The results show temporally alternating coupled and decoupled δ¹³C_carb–δ¹³C_org patterns that extend at least up to ca. 520 Ma. Intervals with coupled δ¹³C_carb and δ¹³C_org data coincide with ocean oxygenation events recorded by redox-sensitive trace elements and/or Mo isotopes. The match of ocean oxygenation and coupled δ¹³C_carb–δ¹³C_org suggest episodic growth and complete oxidation of a moderate oceanic DOC reservoir 6–10 times larger than that of the modern ocean. Following each oxidation event, increased primary production fed by newly available nutrient supply may have led to fast expansion of ocean anoxia, growth of oceanic DOC, and ocean redox stratification, which is supported by up to 10‰ spatial variations in both δ¹³C_carb and δ¹³C_org among shallow- and deep-water sections. In a stratified ocean, decoupled δ¹³C_carb and δ¹³C_org may not have resulted directly from DOC-buffering but from recycling of organic matter by chemoautotrophs and methanotrophs. The upper Doushantuo negative δ¹³C excursion, which is likely time-equivalent with the Shuram-Wonoka δ¹³C excursion, shows large temporal and spatial variability. The real primary δ¹³C signal may be much shorter-lived than previously thought. If the duration of this δ¹³C excursion is within million-year scale (e.g., ≤ 2 Ma), episodic growth and oxidation of a moderate DOC reservoir best explain the coincidence of Ediacaran–Early Cambrian δ¹³C excursions and ocean oxygenation events.