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

Paper No. 280-12
Presentation Time: 11:00 AM


ZHANG, Feifei, School of Earth and Space Exploration, Arizona State University, Room 686, PSF, 550 E Tyler Mall, Tempe, AZ 85287, KENDALL, Brian, Earth and Environmental Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada, CUI, Huan, Department of Geology, University of Maryland, College Park, MD 20742, ANBAR, A.D., School of Earth and Space Exploration and Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1404, XIAO, Shuhai, Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061 and KAUFMAN, Alan J., Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, fzhang48@asu.edu

Reconstruction of ocean redox chemistry during the Ediacaran Period is important for understanding the causal relationship between environmental oxygen levels and early metazoan evolution. Geochemical data (e.g., high Mo and U concentrations and/or heavy Mo and U isotope compositions from sedimentary rocks) provide evidence of extensive ocean oxygenation shortly after the Marinoan glaciation at ca. 632 Ma [1], during the late Ediacaran Period at ca. 560-551 Ma [2], and multiple times during the early Cambrian Period [3, 4]. These episodes of oxygenation may have been separated by intervals of less oxygenated conditions [1, 2]. However, the global redox state of the ocean during the terminal Ediacaran period (ca. 551-541 Ma) is poorly constrained. We address this knowledge gap by measuring carbonate U isotope compositions (δ238U) – a novel global ocean redox proxy – of the Gaojiashan Member of the late Ediacaran Dengying Formation (ca. 551-541 Ma) in South China. An abrupt negative shift in δ238U from values scattering around -0.45‰ to values averaging -0.95‰ (±0.20‰, 2s) was observed in the middle Gaojiashan Member, suggesting a globally widespread expansion of ocean anoxia during the terminal Ediacaran Period. The negative δ238U shift coincides with the onset of a pronounced positive carbon isotope excursion (from 0‰ to +6‰), suggesting that ocean anoxia is the major driving force behind enhanced organic carbon burial that led to the carbon isotope excursion. The widespread anoxia recorded by the Gaojiashan Member is bracketed by known intervals of extensive ocean oxygenation, thus indicating that the Precambrian-Phanerozoic transition was characterized by oscillating ocean redox conditions. The Ediacara biota (ca. 541 Ma) [5] disappeared shortly after the widespread ocean anoxia, suggesting that an expansion of ocean anoxia may have triggerred the onset of a mass extinction in the latest Ediacaran time.

References: [1] Sahoo, et al. (2012), Nature 489, 546–549. [2] Kendall, et al. (2015), Geochim. Cosmochim. Acta 156, 173–193. [3] Wen, et al. (2011), Geology 39, 775-778. [4] Chen, et al. (2015), Nature Comm. 6:7142, DOI: 10.1038/ncomms8142. [5] Narbonne (2005), Annu. Rev. Earth Planet. Sci. 33, 421–42.