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Oceanic anoxia at the Precambrian-Cambrian boundary

¹ Geological Institute, University of Tokyo, Tokyo 113-0033, Japan
² Geological Survey of Japan, Tsukuba 305-8567, Japan

The Precambrian-Cambrian (PC-C) boundary separates fossils representing two discrete evolutionary phases: the Neoproterozoic soft-bodied Ediacarian biotas and Cambrian small shelly faunas. The biological discontinuity is suspected to have been a result of mass extinction; however, recent discoveries of the Ediacarian biotas in Cambrian sediments have led to an understanding that the faunal change was gradual through the PC-C transition. Th/U ratios, which are high in oxidizing conditions and low in reducing conditions, show a considerable positive correlation with ¹³C values at all studied sites of the PC-C boundary. This correlation indicates that reported ¹³C variation across the PC-C boundary from numerous localities corresponds to redox variation in the depositional environment. The negative ¹³C anomaly that occurs worldwide at the PC-C boundary, therefore, corresponds to the widespread development of an oxygen-deficient shallow marine environment. This finding suggests that widespread oceanic oxygen deficiency, which has been interpreted to reflect Phanerozoic mass extinction events, also occurred immediately before the Cambrian explosion.

Key Words: Cambrian • anoxic environments • bioturbation • mass extinctions • Iran

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