--> Abstract: Turbulent Times for Early Animals?, by Swapan K. Sahoo; #90181 (2013)

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Turbulent Times for Early Animals?

Swapan K. Sahoo
University of Nevada Las Vegas, Las Vegas, NV

There is persistent debate about the role that surface oxygen levels played in controlling the diversification of complex life. To move forward our understanding on this topic we have been undertaking palaeoredox work on organic-rich black shales of the Doushantuo Formation, South China. We have found evidence for pervasive oxic conditions, and likely even deep ocean oxygenation at three intervals in the lower (ca. 632 Ma), middle (ca. 580 Ma) and upper (ca. 551 Ma) Ediacaran. However, we have also found evidence for widespread anoxic conditions through much of the Ediacaran. The Doushantuo Formation, hosts large redox-sensitive trace element (e.g., molybdenum, vanadium and uranium) enrichments in anoxic shales, which are indicative of a globally oxic ocean-atmosphere system. However, much of the Doushantuo Formation in basinal sections is characterized by Fe speciation data and pyrite morphologies that indicate deposition under euxinic conditions but near-crustal redox-sensitive element enrichments and positive pyrite-sulfur isotope values. Coupled these proxies point toward a reversion to more widespread reducing conditions during the Ediacaran. Therefore, our work reinforces the emerging view that the Earth underwent numerous rises and falls in surface oxidation state, rather than a unidirectional rise as originally imagined. Further, this framework forces to re-examine the relationship between Neoproterozoic oxygenation and metazoan diversification. Vacillating redox conditions through the Cryogenian and Ediacaran may help explain molecular clock evidence for an early appearance and initial diversification of metazoans but the appearance of most major metazoan crown groups near the Ediacaran-Cambrian boundary.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013