 |
|
|
|
Defining
and Dating the Pleistocene Boundary
Typically, geologists
can identify the boundaries of different time periods by locating
changing rock characteristics or layers where fossils may be seen
for the first or last time. However, the Pliocene-Pleistocene boundary
proved to be exceptionally difficult for geologists to pinpoint. The
rocks containing the earliest Pleistocene fossils were mainly terrestrial
sediments, whereas those in the latest Pliocene were mostly marine.
The rocks at Crotone in Calabria, southern Italy, however, preserve
distinct marine faunas from both time periods. In 1940 the International
Geological Congress decided that the Pliocene-Pleistocene boundary
would be based on the marine faunas located at this site (called the
type section).
Once the boundary between the Pleistocene and Pliocene had been specified,
scientists could calculate its age. This depended on their ability
to date changes in ocean faunas and correlate them with the rocks
in Calabria, Italy.
Dating the beginning of the Pleistocene is beyond the range of radiocarbon
dating and also beyond the limits of other methods that use organic
materials. An alternative method is magnetostratigraphy
(or paleomagnetism). By systematically collecting
and measuring the orientation of magnetic grains in volcanic rocks,
ashes, and other rock types, a record of Earth's magnetic history
can be constructed. New localities can be placed into the context
of this history if they contain rocks with magnetic grains. For the
Pliocene-Pleistocene boundary, biostratigraphic information could
be correlated to a magnetostratigraphic event called the Olduvai Normal.
This suggested that the lower Pleistocene boundary could be no older
than 1.8 million years ago.
|
=
Overview |
Defining and Dating the Pleistocene Boundary |
Pleistocene Glacial Events
Pleistocene Ecosystems and Extinctions |
Human Evolution during the Pleistocene
Department of Paleobiology Home |
National Museum of Natural History Home
Smithsonian Institution Home
|
|
