2004 Articles

A New Period for the Geologic Time Scale

Knoll, Andrew H.; Walter, Malcolm R.; Narbonne, Guy M.; Christie-Blick, Nicholas

The geologic time scale stands as a major achievement of 19th-century science, a coherent record of our planet’s history fashioned from myriad details of individual rock outcroppings. The eras, periods, and finer divisions of the scale not only codify geologic time, they reflect our accumulated understanding of Earth’s past—or at least its more recent past. The Cambrian Period, with its fossil record of animal diversification, began only 543 million years ago (Ma), when Earth was already 4000 million years old (see the figure). In the 19th century and for much of the 20th century, the beginning of the Cambrian (also the beginning of the Paleozoic era and the Phanerozoic eon) marked the most distant temporal reaches of Earth’s tractable historical record. The absence of skeletonized fossils that mark Phanerozoic time made Precambrian rocks difficult to correlate, and so the fine stratigraphic divisions of the younger record gave way to broad intervals that permitted only limited insight into foundational events of Earth history. In 1991, perhaps out of resignation, the International Union of Geological Sciences (IUGS) approved a division of Precambrian time into eons, eras, and periods defined strictly by chronometric age, without reference to events recorded in sedimentary rocks. The eras stuck, but the proposed period names are seldom used. This tradition was swept aside in March this year with the approval by IUGS of an addition to the geologic time scale: the Ediacaran Period. This newly ratified period, which directly precedes the Cambrian, is the first Precambrian interval to be defined according to the principles that govern the Phanerozoic time scale. It is also the first stratigraphically defined new period of any sort to be added since 1891 when Williams divided the Carboniferous Period in two (Mississippian and Pennsylvanian).