寒武纪生物爆发前的死劫难海洋

白垩纪/第三纪界线研究揭示,界线事件留下铱异常和碳同位素异常信息(Alvarez,1980;Hsü et al., 1982)。近来,二叠纪/三叠纪界线粘土中也发现了Ir-δ¹³C双异常(SunYiyin et al.,1984)。

STRANGELOVE OCEAN BEFORE THE CAMBRIAN EXPLOSION

The Palaeozoic and Mesozoic eras were terminated by faunal changes involving mass extinction of the old and explosive evolution of the new fauna, but the fossil record shows only a Cambrian explosion at the end of the Precambrian. Stanley speculated that the explosion was only possible after the ubiquitous algae community had been largely eliminated; ecological niches were thus liberated for explosive evolution. If the Cambrian explosion were preceded by a mass mortality (or by a mass extinction), such an event should leave a record in the form of geochemical anomalies.We have undertaken a search for geochemical anomalies at the Precambrian/Ca-mbrian contact. We report here the discovery of a sharp negative carbon-isotope shift in the carbonate of a clay immediately above a marker in the Precambrian/Cambrian boundary, the China C marker, and interpret this signal as evidence of sudden decrease in fertility before the Cambrian explosion of invertebrate evolution. The discovery suggests that the Precambrian/Cambrian boucdary might be defined by an event-marker at a palaeontologically correlative horizon.