Ancient ocean upheaval marks the spot.

Ancient ocean upheaval marks the spot

Imagine that Christmas did not officially fall on December 25 but that everyone knew the holiday belonged sometime near the end of the year. With some people celebrating on December 27 and others a week earlier, confusion would ensue. In a sense geologists face a similar situation when studying the oldest known animals with skeletons. These organisms began appearing in the fossil record near the opening of Earth's Cambrian period 570 million years ago, but geologists lack an official signpost for the period. Researchers now propose that evidence of an ocean upheaval can help solve the problem.

A committee of scientists is trying to pick a single geologic "type" section that will serve as a worldwide example of the boundary, and most attention is focused on a site in the Yunnan province of China. Once they choose, researchers will need a means of locating the same boundary elsewhere. Normally, fossils can aid in finding a geologic boundary, but the situation is particularly confused at the Precambrian-Cambrian transition, because fossils from that period are not so numerous.

A geochemical fingerprint could come to the rescue, says Martin Brasier of Oxford University in England, one of the committee members. Braiser and his colleagues report the Chinese locale and other sites in Asia share a similar chemical marker. Within the rocks, the ratio of carbon isotopes swings dramatically at the time of the approximate boundary, reflecting a dramatic change in the world's oceans, says Braiser. Although the researchers cannot yet explain the isotope changes, the chemical marker may represent times when the ocean turned over, bringing unusual amounts of nutrient-rich water to the surface. Scientists had previously identified these isotopic shifts in Moroccan rocks. But because the rocks lacked fossils, it remained unclear how the timing of the shifts related to the appearance of certain animals.

Brasier says the isotopic shifts appear to have occurred concurrently from Morocco to Siberia, which apparently sat on the coastline of a giant supercontinent at the time. If so, the geochemical signature will allow geologists to locate the beginning of the Cambrian period even in fossil-poor rocks.