2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 177-3
Presentation Time: 8:30 AM


JIANG, Ganqing, Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, Ganqing.Jiang@unlv.edu

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 δ13Corg and result in decoupled δ13Ccarb and δ13Corg. Recent studies, however, revealed large temporal δ13Corg fluctuations and coupled δ13Ccarb–δ13Corg 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 δ13Ccarb and δ13Corg data from South China are compiled and compared with geochemical data for oxygenation events. The results show temporally alternating coupled and decoupled δ13Ccarb–δ13Corg patterns that extend at least up to ca. 520 Ma. Intervals with coupled δ13Ccarb and δ13Corg data coincide with ocean oxygenation events recorded by redox-sensitive trace elements and/or Mo isotopes. The match of ocean oxygenation and coupled δ13Ccarb–δ13Corg 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 δ13Ccarb and δ13Corg among shallow- and deep-water sections. In a stratified ocean, decoupled δ13Ccarb and δ13Corg 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.