Prominent Isotope Excursions on a Global Scale Just Prior to the Cambrian Explosion

Australia: The Land Where Time Began

Precambrian/Cambrian (Pc/C) boundary sections from around the world have been correlated by the use of carbon isotope chemostratigraphy, the results of which have elucidated significant changes in the carbon cycle that occurred during the rapid diversification of skeletal Metazoa - the Cambrian Explosion. Mainly as the result of the lack of a continuous stratigraphic record, the standard δ¹³C curve of the Early Cambrian has been poorly established. In this paper the authors¹ report high resolution δ¹³C chemostratigraphy of a sample from a drill core that crosses the Pc/C boundary in the area of the Three Gorges, South China. The section extends from the Dengying Formation, the uppermost dolostone of the Ediacaran through muddy limestones of the lowermost Early Cambrian Yanjiahe Formation to a calcareous black shale of Middle Early Cambrian age, the Shuijingtuo Formation. The authors¹ identified 2 positive and 2 negative isotope excursions within this interval. The δ¹³C_carb increases moderately from 0 to + 2 ‰ (positive excursion: P1) near the Pc/C boundary then drops dramatically down to - 7/‰ (negative excursion1: N1). The δ¹³C_carb subsequently increases continuously up to about + 5 ‰ at the upper part of the Nemakit-Daldynian stage. The δ¹³C_carb then decreases sharply down to about -9‰ (N2), after the positive excursion just below the basal Tommotian Unconformity. A primary origin of the record is suggested as these continuous patterns of the δ¹³C shift are irrespective of lithotype. The δ¹³C profile that was obtained is also comparable to records of other sections within and outside the Yangtze Platform, except for the sharp excursion N2. The authors¹ conclude that the general feature of their δ¹³C profile best represents the seawater chemical change that occurred on a global scale. The minimum δ¹³C of the N1 (- 7 ‰) is below the input of carbon from the mantle, implying an enhanced flux of carbon that is ¹³C-depleted just across the Pc/C boundary. At that time the ocean therefore probably became anoxic, and this may have affected sessile or benthic Ediacaran biota. The subsequent δ¹³C rise up to + 5 ‰ indicates an increase of primary productivity, or an enhanced carbon burial rate, which should have lowered the pCO₂ and resulted in the following global cooling. The cause of sea level fall at the base of the Tommotian Stage, which occurred on a global scale, is accounted for by this scenario. Subsequently, the very short term, and exceptionally low δ¹³C (- 9‰) in N2 could possibly have been associated with methane release from gas hydrates as a result of falling sea level. It appears the inferred environmental changes, such as ocean anoxia, increasing productivity, global cooling and subsequent sea level fall with the release of methane, coincided with, or occur just prior to the Cambrian Explosion. The authors¹ suggest that this may indicate synchronism between the environmental changes and the rapid diversification of skeletal Metazoa.