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Paper No. 221-11
Presentation Time: 4:30 PM
LIFE IN OXYGEN OASES DURING THE TERMINAL EDIACARAN EXPANSION OF MARINE ANOXIA
The Ediacara biota diversified ~570 Ma,reached maximal diversity ~560 Ma, and began to decline and eventually went extinct in the terminal Ediacaran Period. The cause of the decline of the Ediacara biota remains unclear, although an expansion of marine anoxia has been implicated as an environmental driver of this event. Previous studies in Namibia have shown that the Ediacara biota were restricted to localized oxic environments, but this life-environment association needs to be demonstrated globally to test the hypotheses about the decline of the Ediacara biota. The Ediacaran Doushantuo and Dengying formations offer integrated paleontological and geochemical data to test this hypothesis. Published d238U data indicate a nearly modern level of oceanic oxygenation as recorded in the upper Doushantuo Formation(d238U= –0.1‰), followed by a global expansion of oceanic anoxia as recorded in the terminal Ediacaran Dengying Formation(d238U= –1‰). Global expansion of oceanic anoxia in the terminal Ediacaran is also supported by a global compilation of redox-sensitive trace metal data. In this study, we analyzed rare earth elements and iron concentration in carbonate parts of limestone and dolostone from Dengying Formation to characterize local redox conditions. The Shibantan Member of the middle Dengying Formation, which contains various Ediacara-type macrofossils and ichnofossils, is characterized by negative Ce anomalies (between 0.46 and 0.63) and anomalously low Fecarb concentrations (mostly<100 ppm), suggesting locally oxic environments despite a global expansion of oceanic anoxia. Thus, Ediacara-type organisms and animals in the terminal Ediacaran Period were indeed restricted to local oxygen oases/refugia during a global expansion of oceanic anoxia. Selectivity analysis of organisms surviving this global anoxia event vs those going extinct will help to resolve the ecological role of anoxia in driving the extinction event across the Ediacaran-Cambrian boundary.